It's not about what percentage is the correct commission for stores but the fact that Apple doesn't allow other stores or installation of apps outside their store.
A publisher can ignore all game stores on windows or macos and still sell their game directly to user if they wanted to but they cannot do that on ios.
That depends on the service provided. In a alot of cases it is good value. That is why companies sell their software via steam and are fine with the App Store. The thing is: you don't have to use steam if it is not a good value for you. But you have to use the App Store with its far-reaching rules. Especially the ability sell your app to iOS users alltogether.
A publisher can ignore all game stores on windows or macos and still sell their game directly to user if they wanted to but they cannot do that on ios.