You ever hear of the Aristotelian unities? Greeks thought drama had to take place over no more than 24 hours. Could be more exaggerated.
Really, though, I guess extreme survival situations, where everyone is under extreme duress, would cause people to bond much faster than ordinary day-to-day living. If two people are fighting for their lives and saving each other, that's going to matter.
Plus, since the audience is watching, since it feels like a long time has passed, the movie acts like it did, whether or not that's true in-universe. Otherwise it'd be pretty monotonous to watch relationships develop in most movies/shows.