[Cheesegear]

Whenever you want to know why somebody did or didn't do something, either
a) It's explained in a book, or
b) It makes a better story that way, one way or another (there are 21[19] Primarchs because that's what they could come up with), or
c) It doesn't matter, it's not important, so the writers didn't bother including it.

Read any book. When a character looks at 'the sky', do you automatically assume the sky is blue? Did the author need to explain that in this fantasy or sci-fi universe that the sky wasn't blue? Probably not. Because it really doesn't matter, unless it does. And, if the sky's colour does matter, it's going to be explained.