They don't own the OS. One of the most powerful forces in consumer software is the ability to set defaults. The vast majority of users will never change the default browser, launcher, or app store; most don't even know it's possible. Google's position of controlling Android allows it to be wildly more corrupt and incompetent than any competing app store and still remain the dominant one. If another store does manage to gain a strong foothold, there are any number of OS changes Google can make to hamstring and stifle the competition. The situation is extremely asymmetric.

Lack of distribution probably.

But Google has agreed to distribute app stores (in America anyway) for a modest $5000 fee + $5000/year fee + transaction fees.

They are hardly mythical, for one I could think of Steam and Epic Games.

Competition is what stops the exploitation.

Epic Games has always offered lower fees and their CEO has been advocating for small developers for years.

There is no reason why Google and Apple get to bundle their stores, and these other big names have to collect the scraps from users who care to look up alternative stores and go through scare screens.