It emerged when people wrote screenplays on typewriters, and there isn't really a good reason to "fix" it. Nobody uses any other format, because a) people who read scripts can make reasonable judgements about timing / pacing etc if you use the standard format and b) writing in a non-standard format is a good enough reason to immediately discard your script and move onto the next one from the neverending pile.

Apparently Mad Max: Fury Road was written as something more like a storyboard than a traditional screenplay. There may be other exceptions, but not many.

Maybe there's a small element of cargo cultiness to it as well. And you can write any old shit and put it in The Format and from a distance it looks like a proper screenplay. But really, there's not much reason to do anything else.