tsadkiel said:
Because let's be honest here - ninjas are mooks.
Well, _sometimes_ they are. Consider, for example, the movie Ninja Scroll. The protagonist Jubei is a ninja, and he is a one-man killing machine. I tried tallying up his kills, but lost count after the first 100 or so. However, many of the opponents he slices up are also ninjas, and they are indeed hapless mooks.
From this, we can deduce that the competence level of ninjas is inversely related to their strength of numbers. A ninja by himself is truly an army of one, unstoppable by anything that mere mortals could bring to bear. Ten ninjas, however, are much less deadly, and could be beaten off by a determined foe. A thousand ninjas are basically comic relief, who can barely keep from stabbing themselves in the foot with their ninja toes.
Ha ha! Ninja toes. In the foot. Ninja TOES. Get it? Get it?
Um.
1 theory is that ninjas radiate an interference field, which interacts with the fields of other ninjas and dampens their ability to act competently. Think of electromagnetism, where a moving electric field generates a magnetic field that acts against the original movement. And just as electromagnetic fields can turn brains into mush (cf the theories on mobile phones and brain cancer), so too does the ninja incompetence field turns their brains into mush.
Hope this helps!