Americans do use "housemates".
In college, a roommate is someone you actually share the room with, perhaps 1 to 3 other people in a dormitory room. If those people shared a house where everyone had their own room, they would use "housemates" to distinguish from the former situation. If there is any kind of confusion like that, people would use something other that "roommate", such as "someone who shares my apartment" etc.
If no confusion were possible, people might revert to using "roommate" but "housemate" could also be used.