To be honest, I've always thought SE has a bit of an obsession with T&A. There are a lot of bosses scattered around with either gigantic (barely covered) boobs, or big ol' bums (also barely covered). Echidna always comes to mind first when thinking of the arses.

Sure, the character models themselves are severely lacking, but I doubt that's because they don't like bums. My guess is that the real reason is probably tangled up somewhere between "flat arses make copying gear between the races easier and will save on dev time" and "server limitations".