Because the tadpole is not a biological parasite, it literally consumes your consciousness and your soul, your body is merely a vessel for an illithid consciousness, that is quite probably poured into you by an overmind.
Imagine a tadpole like an antenna, it hooks itself onto your brain stem and waits for the signal from the overmind. When the signal arrives, the overmind uploads a unit of illithid consciousness to the tadpole and the tadpole transforms the body to reflect the consciousness of a mindflayer. It might sound like a bit of a reach until you realize that the mindflayers can build flesh-airships with psionics. Compared to that, transforming a mere body, even if it's undead, is nothing.
This also fits into the way mindflayers feed. They don't actually need the brain, they merely eat the brain for the psychic potentials it contains, memories, etc.
Ahhh, that's good to know! Ok, then I can rest easy, the antenna analogy is great and makes a whole lot more sense. So you're sort of saying that it works like a type of malware?
So that could still account for incompatibilities or differences in how any given organism coexists with a tadpole (again thinking about that gnoll), but it wouldn't hinge on their biology, then, but one some kind of metahpysics. I do wonder, though: if it's consuming the soul, does Astarion even have one that the tadpole can consume or utilize? Maybe that's the key difference with the dreams—is it latching onto his memories instead of whatever his soul/consciousness might be? We haven't seen another tadpoled undead, have we?
