[1] https://everymac.com/systems/apple/mac_classic/specs/mac_128...
It was also supported in Windows, and given that Windows also kept a separate copy of the menubar as part of every window's chrome, they could have really called it Microsoft Window.
The need for a user interaction method that works at such a low resolution is the origin of Windows users expecting a UI that keeps every Window maximized.
File management in the classic Mac UI was always to open a window for the source directory and another window for the destination directory and use drag and drop to move or copy the files.
File management in Windows was to navigate to the source directory, cut or copy the files, navigate to the destination directory and paste.
Just as the classic Mac OS only maximizes a window enough to show all the contents of the window, but no larger than that to facilitate interacting between multiple windows instead of having every window take up the whole screen.
Personally, I think it goes back to early Windows originally supporting very low resolution graphics cards where you simply couldn't display more than a single widow's worth of content at once.
Remember that EGA was only 640×350. You had to have a user interaction method that worked at such a low resolution.
Having things optimised for drag and drop between multiple windows is the reason for Classic Mac OS to only maximize a window enough so that the entire contents of the window is visible, instead of taking up the whole screen.