Why not? You reach far more people through the stores. Just check the PC gamedev scene. There is basically zero constraints on getting an app installed and use

Why not? You reach far more people through the stores. Just check the PC gamedev scene. There is basically zero constraints on getting an app installed and users are used to it, yet devs jump through hoops to be published on Steam because it gives visibility and access to many desirable features. If only the abilities to use Apple Pay in-app or to use Apple's advertisement distribution system are probably a good enough reason for 80% of the existing apps to continue being on the store.