What makes the menu "physical"? It's a bunch of pixels on a screen. That aside, Fitt's law is this:
Targets can be reached faster and more effectively based on how large a target they are. An edge of the screen is an infinate target in one dimension, but a corner of the screen is an infinate target in two dimensions. In other words, if you're aiming for the top-left corner of your screen, there's no way you can miss by going too far to the top, or too far to the left, since the corner is infinate in those directions. Therefore, the 5 easiest places for your mouse to reach are: the pixel directly beneath it, and the 4 corners. The next easiest places are the 4 screen edges, and then any other pixel on the screen.
So, it doesn't really have anything to do with objects being "physical" or not. Not that a corner is less physical than a menu anyway...
Hope that helps. 😀