"Allow access" is pretty orthogonal, I don't know how it all works in mobile OSes, but I assume everything is virtualized there, so you can't just access whatever you want without user granting a permission (e.g. through a file picker system component).

You can also ship sandboxed apps on Desktop without the store (although I am not sure on how hard it is to auto-update them, usually stores handle that part), at least on Windows and macOS.

Stores handle storing the apps themselves and distributing updates, that part of the cost is real, plus they do manually review submissions (to some degree), but 30% is insane for that.