I've been playing traditional music for decades, even qualifying to compete at a high level at one point. There is a high density of reward inherent in music, combined with variable schedules of reward. There is competition and a challenge to explore the edges of the envelope of one's aesthetic and sensory awareness along with the limits of one's physical coordination.

Many of the same things can happen in sandbox style games. I think there is a tremendous potential for learning in such abstracted environments. What about something like Minecraft, but with abstracted molecules instead of blocks? Problems, like the ones around portraying how molecules inside a cell are constantly jostling against water molecules, could be solved in such environments using design. Many people who play well balanced games at a high level often seem to be learning something about strategy and tactics in particular rule systems. I suspect that there is something educationally valuable in a carefully chosen and implemented rule system.

Also perhaps, it's so much easier to exploit such mechanisms to merely addict people, that overwhelms any value to be gained.