The same way you can run a lot of iOS/iPadOS apps on a Mac since Apple Silicon. Sure, it might as well just dual-boot macOS, but either way it would be really nice to have the option.
That's because MacOS is (approximately) a superset of iOS.
MacOS can run iOS/iPadOS apps because it contains all the libraries and frameworks needed to run MacOS
and everything needed to run iOS Apps (if they are usable without multi-touch, accelerometer etc. & the developer agrees). Even x86 MacOS/XCode could run iOS apps if they were compiled for x86.
To make MacOS apps run on iPadOS you'd have to add a load of MacOS libraries and frameworks into iPadOS.
Why do you need MacOS on a tablet ?
I don't, but the obvious answer is that some people don't want to carry both an iPad and a MacBook. Apple are already selling iPad Pro + keyboard combinations that cost more than a MacBook Pro so how does it make sense that they can't run the same software? They've just launched a very successful MacBook with only an A19 chip, so there's no doubt that the M
x iPads are physically capable of running MacOS when provided with a keyboard and trackpad.
(Personally, I used to lug a PowerBook G3 or similarly-bulky PCs around so
only carrying a MacBook and an iPad seems like luxury, but tell that to kids these days...)
The competition offers 2-in-1 machines, and Windows has a "tablet" mode. YMMV but the bad old days of Windows 8 foisting a touch-centric interface on desktos and laptops seem to have passed.
I think the better question is, why
not let people who want it have the option?
There's the view that it would mean iPad-centric design and UI conventions bleeding into MacOS on "real" laptops and desktops. However, I think that ship has already sailed and MacOS-on-iPad wouldn't make things any worse. Developers (including Apple) can already save time and effort by using SwiftUI and other cross-platform toolkits to target MacOS, iPadOS and others with the same code. Lazy developers won't go the extra mile to tweak the UI for each platform.
The reverse could also happen - if you could just run MacOS on an iPad, would developers still bother to write touch-optimised iPadOS versions of MacOS Apps?
Trouble is, running MacOS on an iPad
without a suitable keyboard & pointer would be a bad experience, and Apple tend to be averse to deliberately enabling bad experiences. Meanwhile, my personal experience of putting an iPad in a keyboard-case is that it ruins the iPad as a handheld touch device. I also, briefly, had a Surface Book with a detachable tablet section - cool, but I rarely used that feature.