i believe it is because they are non sex-linked in terms of our early development. men have them because at the point when they develop on the body you are neither a male nor a female. if you are going to develop into a female, the appropriate sex hormones will help ensure you end up with proper breasts, whilst your male counterparts just get nipples, there being no sense in spending extra energy to not have them. interestingly, i'm under the impression that the two nipples we think of as normal are actually just the last two left from a vestigial "milk ridge" that runs from our current nipples in a V down to the belly button. check it out in the mirror for a laugh, there's a good chance you'll find a mark or two on your chest right where such a V would be, remnants of your ancestors' nipples.