I'm sure the designated hitters who are actually full-time designated hitters are out-hitting the position players. There aren't many of them left. Also that number includes all the NL 1B right?
And the guys who get a turn aren't just corner IF/OF bat types either. Cesar Hernandez has 7 starts at DH this year. He has a .700 OPS.
A lot of players believe you hit better if you also play the field, that it's hard to keep your head in the game just batting. One of the better DHs, Yordan Alvarez of the Astros, has a 1.200 OPS in his 30 LF starts, but just .769 in 87 as DH. (I'm sure sample size and pitchers faced is part of that too).