They don’t make that claim. That’s the service they are selling and the margin they choose. The royalty is similar to software retailer margins going back a long time. Steam charges a similar rate.
It’s pretty trivial to bypass. Just don’t charge for your software, and use the app to access paid resources purchased outside the platform. My company distributes a few dozen apps to thousands of employees, Apple gets $0, because they utilize an existing subscription or license unconnected to Apple.
There are many caveats on this. For example, Apple's terms clearly state that developers must use Apple’s in-app purchase (IAP) system for digital goods or services. So you're restricted to only subscription based pricing. No upgrade pricing. No microtransactions.
Additionally Apple remains the arbiter of what kind of content and apps may be listed. Until recently they were blocking all emulators and cloud gaming apps. They used to block all crypto apps. They still selectively block gambling apps. They still block torrenting and adult apps.
Additionally, Apple is famous for arbitrary and selectively enforced rules. Many developers have their updates rejected for any and no reason after an automated system rejects them for specious reasons.
Additionally, developers are blocked from linking out. They can't up-sell, which is a critical component of a SaaS business model. Nothing in the app can tell users where to subscribe or how to upgrade or change their credit card details. This one alone kills most SaaS applications unless they use Apple's IAPs.
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.
But game developers can sell steam keys directly or cia other resellers and avoid the steam commision entirely. So Valve is goving them a fee service and only chargeing a commision when valve themselves generate the sell.
It’s pretty trivial to bypass. Just don’t charge for your software, and use the app to access paid resources purchased outside the platform. My company distributes a few dozen apps to thousands of employees, Apple gets $0, because they utilize an existing subscription or license unconnected to Apple.