The menu bar is at the top because it’s always been there, but moreover it makes it easier to click menus as you only have to flick your cursor to Y=0 and then move it in the X dimension laterally instead of having to find a small target on a window in the middle of the screen. On other OSs it’s more common to maximize windows.

Distinguishing between processes (applications) and windows just makes sense.

For example say you download something in your browser, then you open a new window, then you close the original window. If windows = processes then closing the original window should cancel the download (because you killed that process), but we all know that would be nonsensical. So user intuition is that there is one main process and multiple windows.

Apple does allow for closing the window to kill the process. However only if there is one main window, such as the calculator app. If there’s multiple windows possible (such as in a text editor or web browser), then closing all windows does not quit the application because the user may want to close a window and then open a new window (e.g. open document or new document).