That is because they are all worried about incoming missles, hijacked airplanes (though I think that was a one trick pony), and such. I think the bit in this (http://www.fromthewilderness.com/free/ww3/031703_rolling_start.html) article pointed out by wrog about the U.S. forces getting soundly beaten in simulation by using cruise ships as weapons, using mosque towers for communication, etc. is a good example. We don't expect that kind of attack.

They just are not expecting a nutcase on a small tractor to drive over with a bomb attached to it.