Reason why this works:

1. You have to keep playing the game otherwise you lose. You can't just stop and go on later, once started, you are invested

2. When you play, you get your reward kick, which is the basis of all addiction. Every small achievement makes you more and more stuck to it. So achievements are small but constant.

3. Other people grow rich when you grow poor. I.e, people steal your stuff, and humans have an instinct against allowing people steal their stuff. So you always come back to watch it