There is precisely zero stopping you from distributing through F-Droid or Amazon Apps or another third-party app store, or even sideloading... Except there's no customers. There's no market. The user's took their freedom of choice, and chose app stores, because freedom is scary and the wild west doesn't get stuff done.
On iOS, which is by far the most lucrative market since Apple captured the high end with a superior experience in the fundamentals (the phones are better, the OS is smoother, etc etc), there is no third-party appstore. None. Will people trade UX for freedom? Absolutely, in the same way they eat at McDonalds because it's better value for money. It doesn't mean McDonalds is the be-all of food, or that we should accept all other restaurants will die and allow McDonalds to run a racket whereby nobody can serve a burger in the whole city.
On Android, half the system is geared towards integration with the Play Store. It's like your fridge only had McDonalds-shaped holes and was preconfigured to order McDonalds for you in a flash. If you wanted to cook, you'd need to order a stove somewhere, sometimes you'd have to learn how to install gas pipes too. Are you ever going to learn to cook, in those circumstances? Fuck no, you're going to click a button and order McD.
There is no market because the platform owners actively ensure that would be the case. They looked at the 90s and thought "Microsoft didn't make enough money, they were too liberal". That's just shameful greed.
