Honestly, we shouldn’t have access to resurrection scrolls, since resurrection magic in dnd is considered high level stuff.

Buuuut that would make people have to savescum more, so it’s understandable they let us res people :'D

Because it’s a concession to gameplay that has no effect on the plot. Why didn’t they use a Phoenix Down on Aerith?

If your party members died in real D&D, you’d have to roll up new characters, and we obviously can’t do that here.

Think it’s a decent workaround that we can assume the parasites can “read” our thoughts and know the plan from the start, so would just stick around knowing they’ll be good.

We can explain it with ingame mechanics and “lore”. The parasite only leaves the body once the body is beyond repair. In 5E revivify only works within 1 minute of a characters death. But the parasite itself is what allows us to use exceed that limit. And we see that with companions, if they are dead dead and you remove the parsite from them, you can’t revive them anymore. Not with a scrolls of revivify nor Withers. It’s not perfect because technically the parasite could keep the body revivable for decades but your character also wouldn’t age.

Only with True Ressurection or a Wish Spell things would get more difficult to find mechanical/lore reasons why this method doesn’t work. But even there you could just say that the parasite has become a part of the person and would simply be resurrected as well.

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.