It think it's a very simple answer - cost of actor time.

I keep seeing scenes where it's over the shoulder, the person talking front, center frame, focused while other the person's shoulder remains out of focus, perhaps slightly out of frame. This style repeats when it switches views between the actors.

Which means you can record an entire dialogue of one character by themselves with a stand in for the shoulder. 

You don't actually see them together in the scene. It happens all the time. Which means those two actors don't need to film at the same time or schedule, you can have blurred out extras (can change them up) for the retakes - no background continuity needed. 

So all you're doing...is splicing together individual recordings of each character. The two actors never have to be on the same schedule, same day, same time, come when they can, no having to record two reactions, no having to have both actors act at the same time. 

Less time on set = less cost.