bc “human” comes from a latin root (homo > humanus > humaine > human) and “man” (and thus “men”) comes from a germanic root (mann > man) 

so you get humans, not humen, since “humans” doesn’t play by germanic rules

look at that i asked a question and i got an answer THANKS

English isn’t a language, it’s three languages stacked on top of each other wearing a trenchcoat.