Anyone can be born with a nice set of tits, but it takes effort and work to have a great ass. Plus the shape of the ass is a better indicator of the general shape of the body.
As a woman, big boobs can be annoyingly heavy and bounce around when exercising. It's also frustrating to find a comfortable bra (bras in general are unnecessarily expensive too). Butt muscles help support posture and add lower body strength. Just a new, un-sexualized perspective for you all ;)