With the menu bar on the top of the screen, it would be impossible to access your application's menus if the focus followed the mouse. As soon as the mouse left the application window, the menus would switch to the Finder or some other app.
I discovered this when playing with KDE, which allows you to select both Focus Follows Mouse and Put Menu Bars On Top of Screen. It just doesn't work at all. I've since trained myself to use Click To Focus and menus at the top of the screen, and I don't really miss the other way too much.