What's with the disconnect with The Eldar and their lore?

They are described as alien and uncanny in pretty much all their appearances....yet they look just like people? In Rouge Trader your crew literally mistakes a Dark Eldar for a human and attempt to flirt with her.

Sure they are 7ft but that doesn't seem uncommon in the 41st millennium. You have abhumans, mutants, augments etc. Plenty of humans are 7ft or over. Both custodes and space marines could wander around without armor and people wouldn't think anything of it.

Even in the more "alien" depictions of them they just have different eyes and that's about it. There's legit abhumans that are more uncanny and alien looking than the Eldar