Because there's no choice you mean? When the developers have choice, on the Mac or on Windows, they generally avoid the store. The AppStore still works on mobile because they force it.
Let's look at something closer to this situation rather than general purpose computers. Mobiles work with different paradigms all around, especially when compared to Windows (or Linux).
There are free alternatives to Google's Play Store and yet developers still choose to pay Google's cut to have access to it. This tells me whatever benefits the "official" app store provides on each platform outweigh the disadvantages (costs) even when free alternatives are available.
> The AppStore still works on mobile because they force it.
Is there any solid reason to believe the AppStore only works because they force it, while seeing the Play Store work just fine even with free alternatives around? And users even go out of their way to get access to it.
That's only because it's pretty hard for the casual user to install third-party apps on Android and even with those artificial barriers, some popular apps are available outside of it, I could mention Fortnite.
How often did you wonder where your car parts come from? How easy is it to mod your ECU? If there are "open source" parts for your fridge? Etc. - insert any topic that's mostly transparent only for the people who actually care about it and dive deeper into the technicalities. Chances are you care less about that but a car tinkerer feels the same way you do about your app store and wonders why you aren't protesting more.
For better or worse your PC is a vastly different beast. I'm not saying it should be like this, I'm saying it is.