I live in America, and yes, we do use "roommates" that way. I don't know why. It may simply be because in America it's very rare to actually share a single room with another person, except in a college dormitory. (Children often share rooms with their brothers or sisters, but we don't call children "roommates." We just say they share a room.) I share a house with several family members, and I usually call them my "housemates," because I feel like "roommate" implies that we're merely living together temporarily for convenience, rather than because we all share the house equally. If we all lived in an apartment, though, I would probably say we were "roommates," because I don't think of an apartment as a "house." (The UK term "flatmates" makes the most sense, but unfortunately we don't use the word "flat" in the US.)

Apparently it is a North American thing: people refer to their 'roommate' when in fact they are sharing a house or a flat together, not just a room. I'm not sure why that would be. In the UK, I would interpret this as meaning you shared a room with somebody.

Thanks for your reply. As my understanding it's just difference between England 🇬🇧 English and American 🇺🇲 English. In North America they call it roommate and in England they say housemate.

I know how you feel - I thought for a long time that Americans like to share rooms! Roomie :) = someone you live with who isn't your family, but in Britain it's housemate.