Of course I could have just Googled it:
"The answer has to do with how humans develop in the womb, said Ian Tattersall, a paleoanthropologist at the American Museum of Natural History in New York City.
"Basically, males and females are all built from the same genetic blueprint," Tattersall told Live Science. "Then, [they] develop in slightly different directions [
in utero and] particularly after we hit puberty."
During the first several weeks, male and female embryos follow the same blueprint, which includes the development of nipples. However, at about six to seven weeks of gestation, a gene on
the Y chromosome induces changes that lead to the development of the testes, the organ that makes and stores sperm and produces
testosterone, according to the book..........
"The fact is that we carry a lot of evolutionary baggage around with us," Tattersall said. "Natural selection is not hovering there all of the time to get rid of things we absolutely don't need."
https://www.livescience.com/32467-why-do-men-have-nipples.html
Namepros - Where you learn about men's nips