
The answer, as we see, is because the Child feels good. A big smile when someone appreciates the Knowledge acquired, the pumping of the fists when an IQ problem is solved; a feeling of Yipee when a Puzzle is cracked and a sense of satisfaction when a Word-game is completed.
Such pats on their back cause a positive reinforcement towards learning, in general, including curriculum learning. The result, often being, better performance in Academics.