Because Apple made the iPhone and they own the iOS store and they decided they could make more money with it this way.
"There are literally hundreds of millions of non-iDevice smartphone users who don't live with these restrictions and who are not being harmed at all. "
This is called assuming your conclusion. There are certainly standards at Google Play and Amazon's Android app store, not every Android user can sideload apps and the idea that no one has been harmed by uncurated stores is strictly false in general and in specific cases like this basically unknowable.
Because Apple made the iPhone and they own the iOS store and they decided they could make more money with it this way.
"There are literally hundreds of millions of non-iDevice smartphone users who don't live with these restrictions and who are not being harmed at all. "
This is called assuming your conclusion. There are certainly standards at Google Play and Amazon's Android app store, not every Android user can sideload apps and the idea that no one has been harmed by uncurated stores is strictly false in general and in specific cases like this basically unknowable.