It's actually not the standard, the standard was iirc 70% for in-store at the time. These days I think it's closer to 50%, assuming no 3rd party losses/licensing.

Nintendo/Sony/Apple/etc are all 30% too, by the way.

The status quo is rarely a good reason for anything

Epic is 12%. Yeah, Epic store sucks and all that. Whatever. There's two marketplaces that aren't first party. One takes 30% and one takes 12%. How is there a standard? You can't look to other markets or other distribution methods to compare it to, because they're all different with their own things.

Edit: GOG is 30% for indie developers (there's a little more to it than that, but basically that). It sounds like with other publishers/developers they negotiate contracts on a case-by-case basis and don't say what they get.