Back in the olden days, long before wewere born, lathes could only cut threads in one direction, i.e. right hand threads. Look at the bolt/dog that a T model Ford has that the hand crank inserts into. When cranking the engine over, the threads tighten each time. As to most people being right handed, well that is true but it only worked out that way because of the threads. Look at any hand cranked engine and the dog will self tighten when cranked.