They have low physical/spatial awareness and body control, which is also why they totter around, bump into things and are generally oafish (the more you use your body the better you are at it).  On top of that they have to seat the weapon against inches of blubber instead of the shoulder, or raise thirty lbs of arm blubber with their pistol.  When you take the shot you want absolute control over your whole body, they do not know what their whole body is doing.