I've never gotten around to actually reading up on this, but I've always suspected it has to do with the frequency of gratification. In real life you could study for 8 hours and, while you'll learn a lot, you don't get that dopamine (or whatever) hit until you complete the test, succeed at the project, etc. Games, however, are constructed so that you get little rewards at regular intervals to keep you hooked, like levels, new gear, etc. Some, particularly a lot of mobile games, obviously prey on susceptible people with that loop, but even "regular" games can get pretty addictive with that sort of progression.

(I'm far from anti-gaming. It's my main hobby. This is just my guess at how the psychology behind it works.)