The problem is less the actual % itself and the fact that it's bundled in with so many things you can't pick or substitute. You can't avoid the app store and you can't avoid the Apple developer tools because they force you to use both of them. So then charging money for something you don't have an alternative to is the issue.

Steam might charge a similar rate but you aren't forced to use them. You can deliver games to PC gamers in a number of ways and many large games opt to not be on Steam.