User blog comment:GrandMethuselah67/Lore Sheet: The Democratic Imperium of Neohumanity/@comment-26322734-20150815225311/@comment-26322734-20150816013109

Humans hadn't quite evolved from Homo sapiens yet, despite their genetic/technological augmentations and their modification of their own DNA, that's still their genus and species. They're still humanoid (a condition that has been kept due to the fact that conquered planets are terraformed to be more friendly to humans), and they're still hard-wired to find other humans attractive. Aliens, of course, are often anything but humanoid, often resembling animals and deep sea creatures.

So when aliens are naturalized as citizens of the U.R.H., they're transformed into more humanoid forms, following the whole Green-Skinned Space Babe trope. This standard of beauty and humanity has actually been enforced by years of culture and tradition (as it usually is in Nationalistic societies) in the U.R.H, so that's why aliens are changed to become humanoid. It's the same reason America "Americanized" cultures back in the 1800's, and why Imperialist Japan took over islands.