Why do men have nipples?
it's not just human men, it's all placental mammals!
pour one out for all of the "I found this weird lump on my male cat's chest, what is it" posts on r/cats
the short answer is: because it's easier.
so, natural selection is lazy. it tends to take the quickest and easiest path to any given solution and can only work with what it already has, so it's really reluctant to drop traits that aren't actually hurting anything by being around! like male nipples.
and since male and female mammals in general start as the same weird little flesh tadpole thing and only start to diverge a couple months into development, it's waaaay easier to just leave male mammals with mostly non-functional nipples than it is to patch them out and maybe bork up something with the female nipples by mistake.
tldr: it's because evolution is naturally lazy.