I don't want to worry about giving my dad an iPhone and making sure he doesn't sideload some scammy app because that's essentially what you'll get. The same was the "third party sellers" on places like Amazon are pretty terrible.
I'm on an Android because I like the freedom. But again, Apple would not exist if you had this rule in place because it would immediately be en-shittified and no one would voluntarily pay the Apple tax that allows them to invest and invent this new ecosystem if they feel they can't control it.
If you don't want walled garden, don't use Apple. Plenty of people don't use Apple products. iOS is about 25% of European market, so what are we even talking about here?