In the US, being roommates doesn't imply sharing the room. (Note: this question isn't about the situation where people share sleeping quarters, such as in a dormitory with separate beds but just one room - the term roommate is descriptive there. It's about the situation where the people don't share sleeping quarters but do share the rest of the house or apartment.)
In the UK, the term "roommate" means a person living in the same bedroom, whereas in the United States and Canada, "roommate" and "housemate" are used interchangeably regardless whether a bedroom is shared, although it is common in US universities that having a roommate implies sharing a room together. - wikipedia
How did the term roommate come to mean someone who shares a house but not the room?
Things I've checked:
An EL&U search threw up a question on the etymology of the -mate suffix; I'm interested in the etymology of the room- part.
I also found a question about using the term roommate in an office setting; my question is about the standard domestic setting.
Etymonline's entry for roommate is rather plain, simply listing room and mate as the roots. The entry for room is somewhat more illuminating, but the noun entry seems to point to a contiguous space, whereas the US usage of roommate relates to sharing a divided space (a house).
So if North American roommates don't share the room, where does the room- prefix come from? The closest justification I've come across talks about shared living spaces: living room, dining room, kitchen, etc, but that doesn't seem to go far enough, except in the generic and unidiomatic sense that everyone who happens to be in a 'room' of any description are 'roommates' - ballroom, restaurant, waiting room, etc.