They didn't. That's just a really awkward way of looking at things. Evolutionary psychology cannot ever supercede basic evolutionary mechanisms because survival is more important that social comstructs unless those social constructs play a DIRECT role in moving one of the four evolutionary gears.

If you want to know about why and how boobs came to be as they are in human females, read at least the first few chaoters of The Descent of Woman by Elaine Morgan. She addresses your question from the perspective of humans as a naked ape, along with a lot of other oddities between us and the other great apes, like why we have sex face to face and why humans grow hair the way they do on the top of their head.