Because it has a weirdly outsized effect on the American film industry (which is a disproportionately massive slice of the global film industry).

Studios factor in the whims of the MPAA heavily when deciding what movies to make and how to make them. Entire movies have just gotten shitcanned and sent back to the drawing board before filming because the script turned in wasn't going to hit the right MPAA rating. Entire genres die out or go to shit when the MPAA decides to tighten up on them (if you remember the plague of god-awful PG-13 horror movies in the 2000s, for example).

It's one of those things that feels goofy to care about, until you actually dig into it a little and realize it extremely isn't.