It’s because it looks like a mouse! Imagine it to have a tail, and think of the buttons as ears, and voila! You have a mouse in your hand! Move it on the table surface or mouse pad, and it seems like its scurrying right?
Of course it’s easy for for our imagination to picture a real mouse instead of the device, but the actual, patented name for the device was “X-Y position indicator for a display system.”… I’m glad it’s called the mouse