I think it's because they're afraid of hurting their pitching arm. They won't use their full force to bat, they have to protect their multi-million dollar pitching hand.
It's not what they get paid to do...best anyway Its funny, cause in little league, high school ball whatever, sometimes the best pitcher is the best hitter too.
I guess if you become a pitcher, it is a position you get "groomed" for and that is where all your energy goes. Some guys then are considered "utility" players, and can play anywhere (with the exception of pitching) on the field equally well. I think those guys get too little attention, and to be able to move about the field and pick up the slack, to me, is a very good player.