There is, somewhere, a physiological maximum speed at which a human can pitch a baseball. Given that MLB velocities have been steadily increasing for decades, we aren't there yet. Once that happens and hitters can learn how to do things other than swing for the fences and hope for contact, they most assuredly will do so. But for now the hitters are still just trying to catch up. So that and earning walks are the most efficient means of scoring runs. Guys who swing for the fences have an overwhelming tendency to pull grounders when they don't manage to square up and elevate the ball...thus the shift.