Love and Friendship.

Why do families stay together for life? Out of familial love, and this is true even if they are not genetically related (adopted children, plus spouses are not genetically related).

Witches can be evil to non-witches, treating them as we treat cattle or swine or chickens or deer or elk: They treat non-witches as a separate and lesser species that the witches exploit for their own ends.

But, just like hunters don't kill their brothers and sisters as readily as they kill their prey, witches treat other witches with respect and a kind of sibling love.

And for a coven, like a family, they engage in mutual cooperation, care, and defense. So sure, they might target a rival witch from another coven, but they feel kinship within their own coven, for the past assistance they all received from each other in getting to where they are.

Don't make your witches so relentlessly evil. IRL sometimes a family member kills another family member, but for the most part siblings can get along because they know each other, had the adventures and trials of growing up together, and survived to adulthood by mutual defense and sharing of resources, with mutual efforts to secure income or resources. Witches could feel this way about their coven, even after they have "graduated" from the coven and are on their own.

Edit: This comment (by Ben Barden) is important enough to add here, so it won't be lost if comments are sent to chat:

The fact that they regularly hunt and murder other people means that they probably don't have many other friends. Humanity is a pack animal. Especially if you live in a world where everyone is out to get you (because you keep murdering them), having friends you can trust is useful, so that you have backup when the witch-hunters arrive.

And perhaps this is true even for the ascended; if not, then the ascended may still wish to protect their "younger" sisters and see them ascend as well. Perhaps the ascended, too, were protected by "older" sisters that ascended before them, and it is part of witch culture to remember where they came from, and that is part of why civilians have such a hard time eliminating covens.

Edit 2: In fact, if this isn't part of witch culture, it makes little sense to join a coven! Because somebody is going to be the first to ascend, almost certainly somebody older, and if #1 is "evil jealous" and ascends, then the smart thing for #1 to do is keep anybody else from ascending; including the #2-9 in their own coven. If all witches are "evil jealous", then as a young witch so am I, so why should I help somebody else ascend and gain the power to kill me? Or the OP's question cuts the other way: Shouldn't us lesser witches try to kill #1 right before she ascends, to protect ourselves? I think a high degree of mutual trust has to be part of the culture of a coven, or it falls apart before anybody ascends.

You Inherit Pieces of the Other Witches

The Aether doesn't just flow directly from the victim to each of the 8 witches individually. While most of it does, some of the aether reaches each witch after passing through a different witch. This means that while a witch will develop special traits and transformations according to their inner selves, these will be slightly influenced by others in the coven--a witch may even inherit a piece of another witch's trait (or a weaker version of it).

This means that a witch wouldn't join just any coven--not unless they're truly desperate, but what coven would want a witch like that? No, covens are formed carefully, each witch analyzing the others, making sure that their inner traits will be mutually beneficial.

This also means that after a complete apotheosis, a witch has inherited a little of the nature of the other 7. Both the completed witch and the others can feel the new connection. They are of each other--they have traces of the same aether flowing through both of their bodies. And how could you kill a piece of yourself?

This new bond makes the witches like siblings, where the complete apotheosic witches strive to protect the others in the coven and help them to also achieve apotheosis.

Magical Upkeep

Even though they are biologically immortal, using their power drains their mana. They'd still need to participate in the ritual to replenish their magic fuel, and hence would need to keep the coven around. Moreover, a transformed witch has the ability to extract life force more efficiently from the sacrifice, even if it is getting divided equally, so keeping more ascendants in the coven would be tolerated.

Introduce Someone Meaner

Witches may be hunted by or in battle with sorcerers. Even an ascended witch may fight only toe to toe with a sorcerer. The existence of a coven may increase their chances of survival, and also promote encouraging other coven members to ascend to fight of the Sorcerer cadres.

Competition

Covens may be in a power struggle within the witch-kind as a whole. This will lead to a system where covens will try to recruit stronger initiates, and have them transform as soon as possible and keep them around to strengthen the coven against other covens.
A coven with a majority of witches fully transformed will of course be more feared than one where only one is.

After decades (centuries?) of backstabbing treachery and murder, a new extremely powerful witch joined the coven. She was so powerful that after a single ritual she achieved apotheosis. (Rumors have it she may have been the last member of a coven back in the Old Country, but there are no survivors from that coven who could confirm this...it's known to have the bloodiest history of treachery of any coven in any country, ever.)

This new witch, who is known simply as The Witch Mother, immediately killed three other witches on achieving apotheosis—and it was clear from the evidence found on the corpses that all three had plotted together to betray the remainder of the coven. They were defeated by the variable factor of The Witch Mother joining the coven, as they considered her only a novice and discounted her from their plans.

Now The Witch Mother keeps ties with the coven for mysterious reasons of her own, and she doesn't tolerate backstabbing treachery. If you achieve apotheosis through her coven, you had better follow her rules—no killing other witches—or she will hunt you down and kill you herself, and absorb your life essence to further increase her own powers. Or she may use you as the next sacrificial victim for the ritual.

The coven would stay together to stave off the dislocation of their minds in the bodies they cheated to acquire. Normally it takes a paragon of virtue 100% attuned to the vision of their perfect self a lifetime of discipline and self-sacrifice in order to naturally achieve apotheosis. Once in that state, their minds and their bodies would be in perfect accord and as they continued to live the body made in the perfect image of themselves would be able to change as well. The witches in the coven, however, cheated the process and used magic to achieve apotheosis. Because of this, their body is in accord with their perfect self at the moment of time apotheosis occurs, but their mind doesn't possess the strength of will, focus or vision to shift their "perfect" image of themselves as they grow and change over time. This means that as their minds changed over thousands of years their body would stay the same.

Imagine a vain witch whos image of beauty changed over time running across another god who is "gasp" prettier than she is... Or a witch who achieved apotheosis in an age where fat was pretty and is permanently stuck with 40lbs of extra girth. Or even a witch who wanted to be young and beautiful forever... only to realize that she is older now and wants to be treated as such.

The solution to this issue is to use the exact same ritual used to achieve apotheosis to recenter the mind of the witch with the ideal version of her self periodically. This makes it important to keep in contact with the old coven and on good terms because, lets face it, any witch you come across has a decent interest in murdering you as part of their own ritual... at least the witches in your coven were able to be trusted long enough to help you achieve apotheosis.

Magical induction ritual

To be able to join a coven a witch must undergo a ritual promise that they will be loyal to the coven. Otherwise the witch is not welcome into the coven.

The terms of the contract state that the witch must always help the other members to the best of their abilities. This magic pact affects witches even after apotheosis. Some pacts may be limited time to encourage new members (and maybe carefully planed backstabbing).

The pact is enforced by magic itself and dire circumstances might befall those who try to break it. Or it may simply be impossible to not abide by it once the oath has been taken.

At the end of the ritual, parts of the apotheosized witch's original body/soul/magic are bound to the witches that conducted the ritual. This creates a magical connection to the other witches where the apotheosized witch can not hurt them. Alternatively, if the witches die that contain the apotheosized witch body/soul/magic, that witch could weaken, lose their immortality, or outright die.

Witch with disciples.

Any powerful enough and smart enough witch has followers - younger witches which just teaching magic. When disciples reach certain level, the mentor could create coven from subordinates.

Since mentor has superior knowledge and power, attempts to kill her are very rare. During ritual, each witch increase his power by same level so mentor still stronger (and much more experienced).
When mentor witch become semi-god, rituals are stopped. On one hand, follower witch never reach the maximum level. On other hand, it would get some assistance and protection given by mentor and before-top level is still very high. Single witch could only dream about such power.

Witch couldn't leave the conclave and establish its own for some reason, or it's very hard to do (mentor witch is one but not single reason)

UPDATE Semi-god level is rare and nobody could say it's a top level. That's why 'mentor witch' don't kill subordinates: probably she could level-up again... if disciples become stronger, or victim would more appropriate.

Why follower withces needs protection and why semi-god witches are not conquer the world, is up to you. In other answers there are great suggestions like rival conclaves or persecution from inquisitors/sorcerers.

Greed.

Why do billionaires continue to expand their fortunes? They want more!

Yes, your witch has achieved an ultimate state. She may be 100 times more powerful than her sisters... but she wants 101. Continuing to participate in the circle will help her power continue to grow, even if not in as dramatic a way as her transformation... and someday she may need to employ this additional power if she is to prevail against another super-witch.