Pitchers don't hit as well as they did in the '50s. With the DH, pitchers don't learn to hit even if they are in the NL. Usually, the best player -- on a kid team -- also pitched. Best athlete I knew was a kid named Ray Dawes. By 12, he had a great curve ball and the hardest fastball in PG County. Great fielder at 3B and best hitter on our team...until we got to 10th grade and he became backup QB and then Post All-Met HS QB in 11th and 12th grades.
The DH rule takes kids and makes them pitchers and nothing else.
By the numbers: check the BA of pitchers in the '50s and NL pitchers today. In 1959, Camilo Pascual hit about .310. In 1925, Walter Johnson hit over .400 and was used as a pinch-hitter. Jim Bunning once pitched a perfect game and also was sometimes used as a pinch-runner. Hank Aguirre, from the Tigers, was such a bad hitter that people used to cheer when he fouled a pitch. His BA was about the same as Gio's.