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.

I mean sure, but even if people have the ability to select a different app store, why are they going to do this? Even if you MAKE them do it, on device setup as was commented elsewhere, do we mandate Apple put the 3rd party stores above their own? Can Apple indicate which is theirs, or does this bias the answer the user gives?

And more importantly I didn't ask what the user is picking or not picking, I'm asking: why is there an app store out there that's better than the main one? Is a 3rd party app store going to spin up a CDN, a transactions system, handle the customer service involved there, etc. and then just... not do all the evil shit Google and Apple are doing now, which is why their app stores make so much money? You're asking businesses to enter a market with established competitors with solid and wildly profitable business models, and to just have them make less money for the principle of it. Which like, I agree with, but you're gonna have a SHORT list of investors if that's your pitch.