Take a look at the videogames landscape on PC. There is an ever-growing amount of game launchers because each company refuses to pay that 30% to someone (e.g. Steam). Having the option to create a store means almost everyone that can WILL create one. Not having that option forces everyone to play by the rules of the existing store.

As a consumer, I like Steam's policies about refunds, I like being able to buy any game with the same in-store credit or the same credit card, I like their client's features like download throttling or scheduling.

I certainly do not like that each half-assed client comes up with a bare bones implementation of the same thing and calls it a day. If Rockstar shits the bed and launches a terrible game, I can refund it on Steam but not on the Rockstar launcher.

Same case for Apple, the minute they allow external stores, half the apps will be pulled from the App Store into their own proprietary store and all the consumer protection would go out the window