Centralization significantly contributes to reducing friction and increasing user trust. Note that even on platforms that do allow competing app stores, market leaders can command similar percentages (e.g. Steam on Windows). This is why.
If Apple opened up the platform, developers wouldn't be paying 5% to Uncle Bob's Discount App Store instead of 30% to Apple, selling just as many units, and pocketing the difference as profit. They'd quickly discover that users preferred to buy from one or two trusted app stores, and that the better move was to pay whatever they had to pay to be present in those stores.
Centralisation, aka natural monopoly.
> And Apple absolutely does increase your reach. Not in any important technical sense, but by creating a low-friction, high-trust environment, which results in a much larger market for apps than would exist if purchasing required users to type their credit card numbers into your web site and download and run an installer.
Plenty of companies will charge 5% or less for low-friction high-trust payment processing, and the auto-install is trivial.