Schools often do a bad job teaching us things that should be good for us, while games do a good job at teaching us things that are often useless.

There are 4 categories here:

Why schools fail to teach everything they intend to:

Boredom is what we feel when our brain decides that there's nothing worth learning. It's the brain searching for new information.

Fun is the experience of developing mastery. When we acquire new skills and recognize valuable patterns, our brains reward us with a shot of pleasurable sensations. 

If a good game teaches you everything it has to teach you before the player quits, then a good school should be one that teaches you everything it intends before the students leave.