In unity I think you can just port to windows mac and linux if you have all of those options downloaded. But yeah, I do see your point. Usually mac isn't known for gaming.
as far as i know, you just compile in windows, for linux you compile in another way, and for mac you gotta actually buy a mac and pay them a yearly subscription fee to actually compile it, it doesnt accept virtual environments. and i believe if people had a button that says "compile for mac" then all developers would have mac versions of their games.