It's less risky to develop some skinner box slot machine because you aren't out much investment if it gets rejected. Plus, the digital goods are pulled from thin air, so anything you sell is pure profit.
Contrast that with a real app that provides a valuable service with backend costs and a subscription fee. There's a lot more investment to build an app like that, so getting rejected is a huge risk and, even at 15%, Apple's cut might be a big chunk of your margin.
Since Apple incentivizes low effort slot machines and detriments high quality apps, I feel like there's been an increasing portion of the app store is turning into what I consider garbage apps.
The subscription cut also creates the opposite scenario of what everyone is always parroting about iOS customers being profitable. They might spend more money, but they're definitely not more profitable (per customer) if you're paying 15% to a middleman.