Why do men have nipples?

The bigger picture more than the fact all fetuses are by default female, the code to turn female parts into male parts happening in utero is, it'll take too much energy to get rid of nipples for no good reason. Men have breast tissue and get breast cancer as well as lactate sometimes. However the genetic code doesn't (normally) enhance that secondary sex characteristic into large, fully functioning breasts for them. And your body isn't in the business of wasting time and energy on getting rid of your boobs. Like the canned goods your Uncle stored in his basement during the 1970s food shortage, it'll take more energy to get rid of them than to just forget about them until he dies.

Have you seen a man without nipples ? That’s why 

To breastfeed in case women become extinct. 

A by product of being adapted from female genomes

keep  them entertained