If you ask anyone that why women have nipples, the simple and straight answer would be that they have them for feeding babies. However, the bodily question that why men have nipples has always befuddled many. The answer lies in early fetal development when the human beings are simply a fetus with an undetermined gender. During the embryonic development, the presence or absence of Y chromosome determines the gender. Up to around six weeks, male and female are basically the same.
If there is no Y chromosome present, the embryo develops to be born as a female. If the Y chromosome kicks in, the embryo begins to develop as a male. However, during this time, the nipples have already formed, so they are simply a just left over. While nipples are of no use to a man, they do not do any harm either. Therefore, I think there was never a reason for the men’s nipples to go away.