The scary thing is that these companies aren't simply being disingenuous - they earnestly believe this.
A company takes for granted that they are good actors, and that their customers' interests are perfectly aligned with their own. With those assertions, increasing their control can only mean a better ability to make things good/safe/simple for their customers.
But that is an authoritarian delusion. Because real difficulty arises out of cross-party emergent complexity - illegible and unmanageable by any single entity. And the road to hell is paved with good intentions. This is blatantly obvious when you, as an individual actor, eventually end up at odds with whatever authoritarian scheme they've implemented - wishing to do something simple that you've personally judged as good/safe, but it's impossible to convince that centralized controller to understand / approve it.
In the real (multi-actor) world, we acknowledge that interests diverge on either side of a transaction. Someone who has bought a printer is then an individual participant in the ink market. Someone who buys a pocket computer wants that computer to act for their own interest - not for it to be beholden to the whims of the company who made it.
Unfortunately, always-on communication, the difficulty of reverse engineering, and overbearing copyright law have allowed these companies to double down on overarching control rather than allowing reasonable demarcation points. Apple could straightforwardly create an app sandbox that would allow running fully untrusted code with fine-grained capabilities, unilaterally design it to not have the vulnerabilities that the Web continues to have (eg fingerprinting), and allow sideloading after appropriate warnings. And lest you think I'm being partisan here, the same exact thing applies to Google's general insistence that sideloaded apps are less safe.
But it's much simpler and more lucrative to double down on authoritarian control until they're forced to create those demarc points, either by direct legislation or by consumer demand - eg if this recent censorship trend eventually pushes them to prohibit secure communication apps in their central stores.