Most likely because they have tested it and realized that the user experience is horrible?
I personally find even the current multi-window experience in iPadOS bad, and wish that I could still turn it off to have the old simplified single window view, since I have a Mac for the complicated stuff and just want my iPad to single task, view media and do what it does best. That seems like a much more simple and obvious request than Apple retroactively adding a non-touch interface OS to the list of operating systems that the iPad will run, but they no longer even allow that as a legacy feature.
I find that the more they merge the devices the less usable they become at their core strengths, as they don't rise to the top of their features, but sink to the lowest of their weaknesses, and just end up as lowest common denominators. And I say this as an owner of Windows touch screen devices, which are okay, but after owning them for some time I find I more often accidentally touch the screen and inadvertently move focus than intentionally use the touch screens as inputs.
But, hey, I used to enjoy experimenting with weird hacks and if they give you the ability to run MacOS on iPads, more power to you. I'd probably even try installing it on my old iPad just to see how insane it would drive me, just for sh*ts and giggles.