Why do you expect things to act like reality, or act consistently, when they aren't real?
Whether something acts like reality is thoroughly separate for whether things act consistently.
Consistency is required for players to make decent decisions about character actions. Inconsistency means players cannot plan intelligently. I expect consistency unless the GM has specifically billed the world as being largely inconsistent.
As for being realistic - this is the most easily grasped form or consistency, for one thing. For another, it is a particular artistic style, and some folks like it. When they game, they expect to see things they like - that is part of the point of playing, after all.
Consider fiction - while some of it is highly unrealistic, a great deal of it is realistic. A lot of folks expect their non-genre fiction to be realistic and consistent, but he story's completely fake. Same with RPGs.
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