It requires effort

The nervous system consumes more glucose than most other parts of the body, and effortful mental activity appears to be especially expensive in the currency of glucose. When you are actively involved in difficult cognitive reasoning or engaged in a task that requires self-control, your blood glucose level drops. The effect is analogous to a runner who draws down glucose stored in her muscles during a sprint.— Thinking, Fast and Slow, page 43

<idle musing>
Maybe that's why some people avoid thinking as much as they avoid physical exercise?!
</idle musing>