It's like having a monopoly in California. "Just sell in New York instead" doesn't work, because you already sell in New York and not selling in California doesn't get the California customers to the New York store.
Compare this to stores that actually have competition, e.g. three grocers on the same street corner. If you don't want to sell to one of them, the customers who want your product just go across the street and buy it there. If you sell to all of them they each represent a third of your sales. If you don't want to sell to one of them, you don't actually lose all of those sales because the same customers can still buy your product from the other stores.
If you don't sell to Apple, none of those app sales go to Android because people don't replace their $1000 phone to buy a $1 app. That's what makes it a different market, which is what makes it a monopoly.