When the iPhone App Store first launched, Steve Jobs claimed[0] the 30% was to cover the cost of certifying software as functional, well-designed, and nonmalicious. Part of it was an ego thing too: he didn't want people fucking up apps and making his pet project look bad, so early App Review focused on a lot of UI polish things in order to make people think iPhone software was just inherently better than Android.

Even a few years in there's already evidence that Apple was entirely aware of how much of a cash cow owning the distribution market for your apps is. There's an internal letter asking about reducing the percentage because someone was worried about the Chrome Web Store (?) eating their lunch. Today, App Review is far too inadequate for the level of software submissions Apple gets, and they regularly let garbage onto the store that's specifically supposed to be curated.

I occasionally hear people complain about how Tim Cook "ruined the company" and that Jobs would never do the kind of control freak shit that he literally pioneered and is literally the selling proposition of the Mac all the way back in 1984. The only thing Tim Cook did was scale the business from "luxury compute" to it's inevitable conclusion as a monopolistic nightmare. The way that the App Store business game is played is specifically that you don't keep spending all your money on better app review. Once you have users and developers mutually hooked on one another, you siphon money out of them for your other projects (or your shareholders).

At one point, you were paying a premium for a better App Store, but not anymore. The business relationship just doesn't work out that way long-term.

[0] I personally think this belief was genuine at first.