system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do men have nipples

I was always wondering.
gpt
Its simple. The default human state is female, so when the fetus is in the womb, it starts developing as if it was a female, and builds nipples. Only later do male hormones kick in and build decidedly male structures.

There is no real evolutionary advantage to NOT having nipples, and they don't cost that much to build, so natural selection has not selected against men having them.