They could but you've got three things to line up then...FX sound, FX image, and the acting. Some real editing to do there. On a big Marvel movie, I bet they could make it look perfect every time, they've got the money. Most other movies I don't think could take the time, or have the money. They use blanks and get it all in one shot. Plus, the reaction is probably much more "real" due to the bang.