Is there any reason why there aren't many alternative shells/wms for Windows and MacOS as there are for Linux. Are there technical barriers, or is it more that the defaults are good enough?
X is designed such that the WM is a user replaceable component with a stable, public interface to the rest of the system. Those other systems not so much.
This is correct. I can only speak to macOS, but window management in this case is actually built on something entirely distinct from the actual system window manager. It utilizes a framework designed for implementing accessibility driven features, but it just happens to provide mechanisms for moving and tracking windows as a part of that.
I myself made one: https://losttech.software/stack.html . You can't completely replace the default one, though. Just override relevant hotkeys and pointer gestures.
I am unsure why do you think Linux has more options? How did you count them?
My guess is that, in general, tiling is less useful in windows because fewer things are command line driven. I'm not saying none, just that in the linux world it is a lot more useful to a lot more people. So there is probably very little demand on the windows side.
