It just kind of depends how thorough the streamer wants to be with their licensing purchase. Generally they just lease out huge chunks of a distributors library in bulk, not individually. So they'll get the cheapest, default cut of the title. More recently I've seen services going out of their way a bit more and having separate listings for subtitled and dubbed versions of non English films, which is cool. But rarely do they get unrated vs rated because if they do they see it as getting charged for the same title twice and don't bother.

Sometimes the cut they get is actually rarer. I had never seen the PG-13 cut of 2002's Rollerball before, because it was a disaster and all Home video releases bumped it up to an R rating to try and help promote it. So if you really want to see the PG-13 cut for...whatever reason, streaming is essentially the only way to do it.