[Sylos]

You can however also argue against having such a global menubar with Fitts's Law, as it means that other UI elements can't be placed at the screen edge, such as the min/max/close-buttons (though there are also concepts where those are global, too) and nowadays also browser tabs.

[oneplane]

While partially true, this could be solved by simply assigning priority to UI elements. Launchers/window managers often list their stuff at the bottom or sides (like docks and window lists) which allow for the same ease of navigation. There is of course only a 4-side box that you can use, and cluttering an environment with bars at all sides doesn't help a user.

The close/resize/etc buttons are indeed a different issue, while the controls are often duplicated (File or window menu on most operating systems and window managers have the close/resize/max options too). At some point I think Ubuntu had a default desktop environment where the close/max/min buttons were added to the top menu bar. I thought it was quite a nice idea, but the implementation never spread to other systems and sadly, support wasn't very universal across all applications.

Thinking along those lines, what if you added the title bar at the top too, you'd end up without a border/bar at the top of the window, making it harder to find something to 'grab' with your pointing device. Some operating systems use a modifier key + the mouse to turn all windows draggable and disable inputs while doing it, but that hasn't had much success (aside from it being the default on certain window managers).