Why don't pitchers throw more curve balls that start at the batter and break over the plate? It just doesn't seem to be used very often.

Right handed pitcher facing a right handed batter or lefty vs lefty, at every level a pitcher with a good curve ball always seems to buckle the batters knees.
I understand they can't do it all the time ( just like any other pitch they have to move the ball, speeds...etc). And I understand the 'they hang it you bang it' but they can also 'bang' a fastball or change up sitting on the inside of the plate.

Thanks