It’s not the same hardware. People get confused because it’s the same SoC, but a computer is far more than its SoC. What handicaps macOS on a tablet, besides a non-touch interface, is the lack of a decent battery and comparatively poor cooling. If people thought the MacBook Air had poor cooling, just look at an iPad. iPad SoC clocks are lower than those on a Mac to prevent overheating. On top of that, the battery is smaller by comparison. IPadOS has a ton of power saving mechanisms completely absent in macOS.How's it not done well? You can hide the left-side mini windows (the Manager part of the Stage, I guess). You can show or hide the dock. You can resize the windows and move them left or right. Pretty much how it works on macOS. Not quite as freeform as macOS, but most iPad apps aren't really written with arbitrary window sizes in mind -- perhaps that will change as developers optimize their apps for iPadOS 16.
I've not played with it too much on an external display to know all of its potential. I know I've done two good-sized windows next to each other, and of course several overlapping windows. Haven't tried to see if it'll do a 2x2 grid of windows, or a 1+2 arrangement, but again, it's not macOS, so the fact that we've gotten this far is pretty good. I'll try it some more later today.
It seems like people are expecting iPadOS 16 and Stage Manager to be macOS. That's clearly not Apple's plan.
Now. whether Apple should put macOS on the M1 iPads is a whole different discussion (one I'm happy to have, actually, because I'd love such a thing, and clearly if macOS can run with awesome performance on an M1 MacBook Air, it could run with similar performance on an M1 iPad Pro with essentially identical internal hardware).
This is why Microsoft has failed with its Surface Pro tablets. They stuck an OS that is optimized for a desktop onto a tiny form factor with a tiny battery. In order to make the battery life halfway decent, they had to use crippled CPU’s and no dedicated GPU. Basically what they produced was a mediocre laptop and horrendous tablet. That is what the iPad would be with macOS on it.