If I had to come up with a justification for that it would probably be that the whole point of a Houri is that they are unnatural. Unnaturally beautiful. Unnaturally long lived. Even their parentage is unnatural as it's apparently extraordinarily rare for a Nymph and an Elf to produce a child.
On the meta side of things, from what I was saw while reading up on Houri, they seem to be somewhat of a Mary Sue race, though I could be wrong there. I think the whole point is that they're supposed to be these eternally perfect and pure things.