Think about it from Apple's point of view. If they make app installation easy from anywhere, you soon end up with tons of ads, popups etc. that direct you to a crappy app's site, which is either just very badly designed or straight up malware. And the average user will download them, just like they have been doing it on Windows (ever looked at non-technical people's computers? Remember toolbars?). Then people end up complaining about the manufacturer of the computer because they don't understand the difference. Windows used to be blamed for being slow after people installed all sorts of garbage on it. Similarly, in the minds of non-technical people, Apple would be blamed for making their iPhone experience clumsy and frustrating. Apple support will be called when they lose data or get their nudes stolen.

Apple wants everyday users to have a streamlined, simple, "just works" experience.

It is a common principle in many other facets of life. You can't just buy any prescription medicine for example, even though theoretically a well-prepared patient with access to the newest medical literature could figure out what he needs. But the vast majority most definitely cannot and relies on some kind of access-filter.

No one is forcing you to buy an iPhone or use iOS. I'd say the market has spoken on the whole curated app store thing, and it largely doesn't give a hoot. There are entire phone OSs built for people just like you. Vote with your feet and your wallet.

It would be if users didn’t pay real money to voluntarily opt in to the scheme.

Devices that can’t (generally) run malware are a value, to some. I have computers, and I also have Pixelbooks and iPads. I use them for different things, and I store different data on them.