Muffit:
My dear Muffit, it was a cruel joke played on men by the Creator. What man wouldn't want his own rack on his own chest to enjoy anytime he was dateless and wanted to enjoy a set of boobies? So the Creator gave man nipples to tease him and make him wish he had the rest of the package that was denied him.
Nah, that's not really why. It wasn't a joke played on man, it was a hint to serve as a suggestion to man that if he wanted the rest of the package he would have to connect with a woman in love and matrimony to get the rest.
Either answer will work, from a certain point of view.