I think this comes down to Apple's envisaged window management paradigm being centered around full-screen windows. In Apple's UX view, it seems like you're meant to maximize windows to "spaces", and switch between workspaces using a few gestures (4 finger swipes mainly) to reveal all windows, and swap between workspaces that way. Rectangle, as others have pointed out, give you the "snap" experience you would expect f…
MS did it because.. I think Windows used to only support 1 window per program or something. That's why old versions of Office basically go full screen and each document you open is another 'child window' of the main office window. Same was true for stuff like America Online lol.