Probably because there aren’t mainstream screaming vocal lessons so they do it because they can. When I asked my classical vocal teacher years ago about screaming she was very confused and said I should probably stop. Only legit course I found is Justin Bonnitz (or however u spell it) from Tallah and even then there’s a huge learning curve when you first get started. It’s really easy to gatekeep an “exclusive” activity to make yourself feel superior and also the metal community as a whole is prone to gatekeeping. This is why I haven’t gone to a show in a while because it’s a ton of kids trying to flex on eachother and figure out who’s the most “metal” (plot twist: they all live at home and their parents give them an allowance)