This is just my guess (I must solidly highlight that I haven’t played the game), but in normal D&D conditions its because you don’t survive Illithid tadpole implantation. Within about an hour or two you’re dead, and even if you kill the parasite before it completes its job you’re left pretty much brain-dead without a higher level Restoration or Heal spell because they directly eat gray matter.

They don’t just plug into your brain, they eat the gray matter and perform some sort of biological grafting between themselves and your nerves, then generate some sort of alien chemicals/hormones to morph the body over a week or more. So, despite BG3 letting you live, likely due to some sort of plot complication I’m unaware of slowing things down, killing the host isn’t likely to drive the parasite out of the body until its finished chowing down on your brain matter. (Even wild tadpoles left unattended eat brains until they grow into monsters.) Either way at this point you probably can’t actually be resurrected with Revivify, you’d need higher level Resurrection because your body isn’t fully intact anymore. (I mean, maybe you could but at best you’re in a brain-dead vegetative state with 1HP because Revivify doesn’t replace lost parts.)

Also a good chance the creature only abandoned the other host because they were already close to being out of brain food or something weird. Like them psychically being forced/ordered out of the host.