Microsoft didn't make the computer, and has no incentive to ensure a high quality UX.

Apple cares about their hardware customers and fosters a well-earned trust. The AppStore rules are a huge part of the foundation of that trust.

But Microsoft made the OS, hence why shouldn’t the user trust their neatly-integrated browser? The analogy still holds.

Apple is a hardware company that wants to ensure the best possible UX for their customers, and does so by vetting the software that gets installed on their hardware products.

What's Microsoft's parallel motivation in your analogy?

So you're saying that a software company that makes operating systems, and has the incentive to want programs that run well on the said OS, has no interest in wanting its users to be locked into the browser they also make?