Most single player games are not "true games". They are puzzles. This is also how players perceive them.
As defined by Chris Crawford a game requires competition. Multiple agents who play, which creates the possibility of win and loss.
In most single player games, the game agents are not equal competitors who can win. They are only obstacles and challenges for the player. Even if the game is built with difficulty in mind.
So the mistake of thinking of actually intelligent game agents is like thinking why doesn't the Rubik cube ever win. It's not why someone plays it.
Notable exceptions are where software is used to substitute a player in a multiplayer game, where they do usually maintain the game structure and compete.