Well, no, it's not about the Operating Systems; it's about the UI paradigms themselves.

I believe that Apple is worried that the desktop WIMP UI itself is going to be disrupted and fade into irrelevance due to 1. "do everything on the web" devices like Chromebooks, and 2. VR/AR productivity-workflow paradigms that are yet to be formalized; and so they're trying to find a successor to the WIMP UI, one that will still be relevant in 2040, even if/when the WIMP UI dies.

The distinction between iPads and Macs isn't arbitrary, insofar as iPads don't require a keyboard, and Macs do. iPadOS (and iOS) apps have to be designed under the assumption that a keyboard is optional; and that really changes things about how an app can work. You can port apps designed for keyboardless tablets to macOS just fine (and as of the M1 you don't even have to, you can just install them); but fully-featured macOS apps can't just be thrown onto a keyboardless touchscreen. (And you can't say "well, you can't install 'keyboard required' apps if you don't have the keyboard", either; the OS has no idea if the user owns a keyboard but just hasn't bothered docking it.)

You are accidentally correct in the other direction, though — that there's no reason you couldn't run iPadOS as well as macOS on a theoretical "touchscreen Mac" (which would be a different thing than an iPad, precisely because the keyboard would be welded onto it, and so apps could guarantee its presence/require you to use it.) The reason that Macs don't have touchscreens, AFAICT, is because Apple wants to use the iPad — along with everyone who buys one, and every developer who signs on to develop apps for one — as an isolated laboratory to run this "successor to WIMP UI paradigm" experiment. They don't want to "dilute" that experiment by allowing those users and developers to get the advantages of the iPad from any of their other products — because then those users and developers wouldn't be incentivized to use the iPad, and thereby to give them the experimental data they need.

Well, we asked ourselves that at the launch of the iPad. At the time it was basically a reading/web browsing tablet (in other words, revolutionary). But people had grander visions, like running DAWs on it and porting Photoshop and developing software. All of these things are not hardware-limited; their exclusion entirely boils down to arbitrary software decisions made by Apple.

So, we waited. We let it grow for more than a decade. What we have today is just a bigger version of iOS, which is a reflection of Apple's refusal to upset a paradigm they directly profit off of. They're genuinely incapable of disrupting the computing market, because they're the ones abusing the market the hardest.

My only hope is that legislation steps in to stop all this bullshit. Your customers shouldn't be treated like guinea pigs, and they should have the authority to install whatever they want on the hardware they own. If Apple can't design a product that respects those two simple principles, then they're going to have a hard time courting modern-day pros that use Macbooks and Wintel machines.

VR/AR makes people feel sick and are fundamentally uncomfortable in a way a screen not strapped to your face isn’t.