Why? The belt is supposed to go across your chest and mine does that very well. If it was across my throat it would decapitate me in the event of a wreck. I'd rather have my shoulder dislocated than my head chopped off.
Because the forces of an accident will propel the belt down and you forward if it's under your arm. Watch some crash test dummy footage. The belt is designed and instructed to be worn 1 way for a reason.