I mean, presumably because it’d compromise their vision for the game or some such? Some games use gameplay as part of the storytelling, so nontrivial difficulty swttings would compromise the story being told (for example if the game wants you to experience a gruelling trek through a hostile area). Now that doesn’t mean a story mode or similar is bad, but there are reasons to consider for a game dev to consider such settings incompatible with their game. Also in a game with more complex mechanics difficulty would be more complicated than player and enemy stats, and a dev might simply consider implementing satisfactory difficulty settings not a good use of their time.