I'll tell you honestly why. It's because if they charged 5% or 10% it wouldn't be worth Apples time.
If they can't make 30% on the app store they'll go do something else which will make them 30%+ and leave it to rot. Because 5% or 10% is a commodity business and they can do better than that.
If there was something else that they could be making that kind of money for that little work, they'd already be doing it. Apple got lucky with a lock-in ecosystem and they obviously design great hardware, but I don't see them just up and deciding to dominate another market to make similar profits just because they're Apple.
Above something that is probably quite marginal this is basically free money for Apple. What are you comparing 30% to? Profit vs gross income? That's not what the 30% are here. That's 30% of an amount which depends on:
* a market that is so large that in a bunch of area it is basically not limiting - Apple has an influence on that but capturing so much value on 3rd party apps should not be warranted IMO, otherwise it would be warranted for them to also capture a big part on most of software for MacOs sales (esp. since the configuration by default is now quite secure so the argument that the App Store for iPhone is so valuable because it is curated is getting weaker) and for MS to capture a big part on most of software sales for Windows (and maybe give some crumbs to PC hardware manufacturers)
* the success of 3rd party apps, most of which has not much to do with Apple
Well, the Mac software landscape flourished before Apple had an app store. I wouldn't mind if they just gave up and let people distribute software how God intended.
If they can't make 30% on the app store they'll go do something else which will make them 30%+ and leave it to rot. Because 5% or 10% is a commodity business and they can do better than that.