And since movies are over a century old every country has a similar regulatory framework.
Most countries do not have an equivalent for apps/games and so it is ultimately left to Apple to regulate.
Also aren't movie rating systems like the MPA self-regulated? Not required by law. There's already equivalent systems for video games with PEGI and ESRB.
Still a bit flawed though: I thought that there is already a ratings board in America for software?
Every country has regulations for music/movies/TV. US and a handful of other countries have them for games and I am not aware of any for apps.
https://www.esrb.org/ratings/34178/Minecraft%3A+Story+Mode+-...
Meanwhile Netflix's own Bandersnatch is not ESRB rated, and isn't too different as a game. Yet Apple has supported both of these perfectly fine regardless of their lack of proper ratings.
We literally have ESRB that's a whole lot less arbitrary than Apple's review system.