already in X11 time, compositor and window manager were one unit: https://en.wikipedia.org/wiki/Compiz

why? because the compositor completely changed the geometry of the desktop. being 3-dimensional. now, today most wayland compositors are 2 dimensional, but they don't have to be. a window manager that works with a 2 dimensional compositor would not work with a 3 dimensional one and vice versa. the window managers listed in the article are only compatible with this particular compositor.

it seems to me that separating window managers from compositors from the start would have created the expectation that all window managers work on all compositors. that is not and will not be the case.

if all compositors start separating out the window manager then the result would be that as a user i now have to choose two components, a compositor and a window manager, and i have to make sure they are compatible.