Because pitchers are drafted for their pitching ability. Everybody else on the field has to be well-rounded. Every other player has to know how to play defense and how to swing a bat. Pitching is such a rare talent to make it to the big leagues, that they really don't do much else. They just pitch. Sometimes you'll get a guy who has a good glove, but for the most part, they're not in the big leagues because they have quick reflexes or because they can hit the deep ball. They're in the league solely off of their pitching ability. A pitcher will almost always let the professionals field a pop up.