Weird how they lump cosleeping as both the same surface and same room. Very different situation for the safety of the child, especially when they are under 2.

They do that because in both cases, the child can hear the breathing of the parents so is unlikely to succumb to the part of SIDS where they stop randomly breathing during the night (happens to all children as brains learn to brain) without restarting (this part doesn't happen to all children). Apparently hearing breathing is good for reminding a new brain to do the breathing thing.

That said, bedsharing and roomsharing have been studied separately under their own titles.