There are two reasons for mental fatigue as best as is known/we can infer. The first is stress from the task (by which I mean the immediate thing that is correlated with cortisol levels). The second is that maintaining attention on a task, where there is not much reward or feedback on the task's progress makes maintaining attention harder and harder to do as time progresses. Imagine a very heavy door that wants to swing back shut the longer you hold it open. Resisting this too is linked with stress inducing mechanisms.
A good theory should explain why watching a television serial, a very complex task involving long term memory retrieval, short term memory of details, inferring various motivations, theory of mind (on a meta level too via genre savviness), agent modeling and prediction, language, visual, audio is less taxing than solving 1000 simple arithmetic problems. The resource management and attention based theories have better explanations for why that may be.
A good theory should explain why watching a television serial, a very complex task involving long term memory retrieval, short term memory of details, inferring various motivations, theory of mind (on a meta level too via genre savviness), agent modeling and prediction, language, visual, audio is less taxing than solving 1000 simple arithmetic problems. The resource management and attention based theories have better explanations for why that may be.