If you are trying to hit a moving arm or leg, there is a very good chance that you are going to miss. And on the off chance that you don't miss, it's likely that the person won't be completely incapacitated. Then what happens? Stab stab stab

The torso is the largest target on the body and it is relatively stationary. And getting shot in the torso is going to put somebody out of commission (but not always dead)