Another reason is that a/c would be sucking in the debris on a powered push back and I would suggest that it might be considered very unsafe at most airports. As someone said a pilot cannot see behind him, yet. I believe that a number of A340/330 do have this facility. Boeing is at present developing a 767 that has electric motorised nose wheels, thus the a/c will drive itself out and may be able to taxy for some length reducing amount of fuel used and perhaps saving a lot of noise adjacent to the terminal buildings.