I think the OP already covered that:
You mention them, but you were focused on "answering the question". Which, sure, they are there to help do that. And I understand that your focus is generally on narratives and processes, but that focus seems to ignore direct engagement with player intrinsic motivations. Game reward dynamics, and player emotional engagement with the game and play seems largely absent in your reasoning.
So, you seem to entirely miss another, more basic answer, "Because playing with the rules IS FUN (enjoyable, entertaining, or whatever similar word you want) in and of itself."