bigbet,
First, props to your donation.
Hitting a baseball is the hardest skill to achieve in any sport. The game is a game of failure. You succeed 3 times out of 10 and you are an all-star. Hitting is very very hard. You need to be extremely talented, like winning-the-lottery talented....you need to practice, and you need lots of experience. Baseball is a very competitive sport. It has minor leagues. Triple A, Double A, Single A, High A, Low A , short season, rookie ball, instructional league, independent leagues. Those are all leagues, not teams. The hitters in the major leagues are the best that the world has to offer. And then you compare a pitcher to the best hitters in the game, facing the best pitching in the game.
To answer your question:
#1 - Lack of Practice. They get paid to pitch, not swing the bat. Any effort devoted to hitting, is effort away from improving pitching.
#2 - Lack of Talent. It is SO HARD to break into the big leagues as it is as a hitter. There are outstanding hitters in the minor leagues that may never make it because they are in the redsox or yankees farm system, and those teams just sign big name guy after big name guy. Some just never get the chance because of politics. Most dont get the chance becase it is very difficult to make it as a hitter. Now imagine those hitters, and how good they are, are much better than these pitchers, that dont practice hitting, and werent as good to begin with.
with all that said......the real reason though....is
#3: at-bats. There is no substitute for experience. From high school to college most pitchers dont hit. The maximum number of at bats a pitcher may have before he breaks into the minors is 300.
in the minors he gets 50 starts and maybe picks up another 150 at-bats. Now the pitcher has got 450 significant lifetime at-bats under his belt, and he's not even that good of a hitter to begin with.
Over the course of just one 162 game season, a regular everyday hitter will pick up 400-600 at bats---which is more than most of these pitchers are getting their entire lives.