As a system macOS definitely has some holes in keyboard navigation, as well as a couple corners which are KB-navigable but the way to focus them is not immediately obvious.

On the app level, keyboard navigability depends on the developer. If they take the time to dot their I's and cross their T's it's between decent and great, but if they don't care it'll be bad.

To reinforce your point, by default on macOS you can't use Tab to select different options in a dialog box. You have to enable that in System Settings -> Keyboard -> "Keyboard navigation" which is off by default.

My theory is that Mac was born with a mouse while Windows (and Windows apps) originally had to function on machines that might not have one. Thus, keyboard navigablity was prioritized, and that design sensibility stuck around longer.

> Windows components however, especially old ones, are incredibly accessible

Bing, Bing, Bing! I came here to vent how the new windows 10 and windows 11 interfaces (made by Mac enthusiasts, ughh) have omitted keyboard accessibility for a cleaner look. They also dropped drag and drop support (which I never knew about in windows 7) and I completely missed how useful it could be on Windows 10/11.