They don’t have to. For about 50 years the American League had the DH rule, which allows a different guy to bat in the pitcher’s spot. Technically this is an option but until Ohtani it was an option that was literally always taken, because until Ohtani there hasn’t been a player who is world class at both pitching and hitting in over 100 years. The National League required the pitcher to bat.

As of a few years ago both leagues have the DH now.

MLB also recently implemented an “Ohtani rule” which basically allows the pitcher to DH for himself, meaning he can remain in the game as the DH once he is no longer pitching. Previously if he batted for himself, that would mean the team had chosen for the pitcher to bat instead of using a DH, which is a choice you can’t go back on, so he would have to be removed from the batting lineup once a new pitcher replaced him.