Just because what it produces is in focus and at the centre of the field of vision, it is more easily seen. This is one reason we are so much more aware of what it contributes to our knowledge of the world.
A play, speech or any form of argument is effective in its focus but neccesarily leaves out other perspectives - the inherent heirachy is neccesary for movememt.
the thumb infront of your face or the window behind it
the impossibility of covering a distance by halves or the reality that we do it every day
