I suppose they are a bit like Farmville. The game needs constant nurturing to get the best out of it. If you dont play it within a certain timeframe bad things start to happen. In these farming type games, crops die if you dont log-on and harvest them.
It becomes an obsession with some. Even though I'm an avid gamer I tend to prefer games you can have a 30 minute blast on and then leave. These games can become an obsession for some, the constant draw to improve what you have built on.