There. That's the reason why. That game sounds boring as shit, and no one would buy it.
Learning in a game has to be implicit, if you make it explicit users will sniff out your agenda and run away. For example, you can't just make the gamer write an essay about Shakespeare to open the treasure chest, but if you somehow included subtle Shakespeare references in Zelda that could work.
And with that in mind, maybe games are already doing this. You're certainly learning something about economics in Roller Coaster Tycoon, and I definitely had to flex my brain muscles for some of the puzzles in Zelda. It has to continue to be implicit though.