I understand the Fitt's Law concepts behind a top menu bar, but I wonder if this is a scenario with moving goalposts.

On a 1984 Mac, you had like 512x384 pixels and a system that could barely run one program at a time. There was little to no possible uncertainty as to who owned the menu bar. (Could desk accessories even take control of the menu bar?)

But once you got larger resolutions and the ability to have multiple full-size programs running at once, the menu bar could belong to any of them. Now, theoretically, you should notice which is the currently active window and assume it owns the menu bar, but ISTR scenarios where you'd close the window but the program would still be running, owning the menu bar, or the "active" window was less visually prominent due to task switching, etc.

The Windows design-- placing the menu inside the window it controls-- avoids any ambiguity there. Clicking "File-Save" in Notepad couldn't possibly be interpreted as trying to do anything to the Paintbrush window next to it.