When humans were being created:
"Alright so, let's see the blueprints for the humans."
"Okay boss, here we got the man & the woman."
"Mhm mhm... Hey John, what's the on the woman?"
"Oh sir, those are nipples, we gave the woman that so that way she can provide milk for the baby if she ever reproduces."
"Mhm mhm, alright. Give nipples to the man as well."
"Wait what? Sir did you jus-"
"DID I STUTTER JOHNSON!?""
"No sir! Right on it!"
And that's why men have nipples.