Might have somethng to do with the speed at which it is prepared. Or maybe the rapidity at which it is scoffed down as your walking back to your car. It certainly has nothing to do with a 'fast,' as we refer to a break from eating to rest our internal organs.
Quite frankly, I haven't a clue why it is called 'fast food' except perhaps as a gimmick to get people to buy it. Whatever the reason I feel I must be a little judgemental here and say that it definitely detracts from an easy-going layback life style. Give me a liesurely lunch or dinner with friends over a table any day.