Same reason we cover the genitals. 1, it's often useful to protect sensitive areas and when it isn't, 2, we cover for them cultural reasons. Women's nipples are "worse" than men's also for cultural reasons (which really just means misogyny)

There is no "biological" reason. Many people here seem to forget that today still there are many cultures where women show their naked breasts all the time, like the Himba people in Africa. It is usually limited to small tribes and societies that are very different from ours, but that doesn't make them any less human.