Because you can't do that in Major League Baseball.
Quote:
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The Designated Hitter may be used defensively, continuing to bat in the same position in the batting order, but the pitcher must then bat in the place of the substituted defensive player, unless more than one substitution is made, and the manager then must designate their spots in the batting order.
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If you move the DH to the field, your pitcher needs to hit somewhere in the lineup. I don't know if OOTP would allow the pitcher to be moved into the lineup in the middle of a game.