Lol, there's a book called "Why Do Men Have Nipples? And Other Questions You'd Ask a Doctor Only After Your Third Martini".
It's something like in the womb, all fetuses, babies, embryos, whatever, are girls up until a certain point. Then sometimes mail chromosomes kick in and the fetus grows a penis, testicles, etc. And so, since the fetus always starts out as a girl, it grows nipples, which girls need, of course, for their breasts that they grow in puberty. So there's your answer.