If you try to take your business elsewhere, you have no market, because no one else is able to create a store which targets the same market. This is entirely unlike a grocery store, because if Safeway is unreasonable you can launch an entire store just for your own products and people can shop at both.
However, with these curated store platforms, a user can only use the App Store. They can get a different phone--at ridiculous expense, as there are numerous anticompetitive lock-in mechanisms with respect to licenses that prevent this: I would, for example, lose access to all of the music and books and apps I've ever bought--but then they aren't able to use the App Store anymore.
The problem is that virtually all sane people only carry around a single phone. This is not true of grocery stores or frankly any other form of store unless you map the analogy to geographic regions, where people have to move to a new house in a different community--at great difficulty and expense--to get to another grocery store. Even if you only have a monopoly over one "small" region, that's still a monopoly.