It goes deeper than just simple engineering though. It affects tone and overarching game design. It is multiple extra dimensions that have to be considered across every aspect of the entire game. If it is done poorly, you get paper dolls on easy mode and damage sponges on hard and nothing of merit to compensate for these facts. The difficulty of the game goes from being genuine to artificial.
I don’t need a game to be hard, I need it to be consistent and well thought-out. Animal Well for example is a rather easy game, but because it only has one difficulty, the developer was able to keep a very tight focus on the world and puzzle design. Everything is layered there, because they don’t have to be containerized and sliced into pieces to account for adjustable difficulty settings.