Apple created an integrated product that billions of people prefer. I don’t think it’s for us to arbitrarily tell Apple which parts of their product have to be split into different categories we arbitrarily define for them.
The only way to install an app on an iPhone is via Apple's store. You can't download it from the dev's website and install it. You also can't use a different app store because Apple doesn't let you. And you also can't buy something on an app (eg: a subscription) without using Apple's payment system or even link to a donation page (why?).
You can have a well integrated product and still give users and developers some freedom.
For example, Android phones usually come with Google's Play Store, which is what the vast majority of users use. It's no different from Apple and the App Store. Then, optionally, you can download the apk (the .dmg or .exe equivalent) from the dev's website and install it. You can also install a different app store, if that's what you want (almost no one does it, but the option is there). Apps are still only allowed to do what the OS let's them do (eg: you still need to give them permission to access your location, camera, etc), so you're only missing the privacy given by the review process. And when it comes to payments, you can use Google's payment system or something else. Not very different from what happens on your computer.
Some people say this is terrible because then Facebook can create their store to bypass Google's rules. In practice, Facebook knows most users won't sideload apps or install a different app store, so they follow Google's rules like everyone else.
Because we have a regulatory system designed to ensure that happens as capitalism doesn't really function if you don't.
Also "platform" is bullshit. By "platform" we mean "Apple used it's control of the hardware to force control of all software on an unprecedented level so they could rent seek." There is no right to a "platform" here. Apple run an app store. People should be free to chose to use it or not, but they're not because Apple abuse their position as hardware manufacturer. That isn't a platform.