For example : Megadeaths happen with nary an eye batted in Japanese movies, but you rarely see that kind of thing in the American. And the total dominance of the aristocracy over the underclasses is as assumed and invisible as gravity in French movies, but it seems to be taboo elsewhere.
I think there’s a way simpler economic reason. If you have an ensemble of actors, the more scenes they have the more you have to pay. So killing them of one by one is an easy way for a studio to save money (let’s not forget, the US has an Union for their actors). Also we live in the age of sequels so having only a minimal cast transfer between movies means way less contract renegotiations. At least I think it started that way and then just grew into a cultural school of filmmaking in Hollywood. It just became the way you do things.
It might also be an artifact of the popularity of slasher movies from the early 2000s’ where the whole point was to kill characters.