For the same reason they have brains, hearts and arms. Nipples are part of the human genetic blueprint, says Cornell University comparative embryologist Drew M. Noden. Saves on parts.
Nipples kick in around the fifth week, well before the great sexual divide.
In a few cases, adult males subjected to high-enough doses of estrogen have actually lactated. The nipple saga hardly ends here, adds Noden. Many mammalian species, as you know, have multiple pairs of nipples running along mammary ridges on the chest and abdomen. So better shuffle on to your next romance prospect. The calculating lady loves you not.