It's just that the game play is extremely addictive that we all want to play it again and again. Especially when you're in the lower levels it is quite addictive to just keep on doing the jobs again and again in the struggle of improving your level.
Moreover, you also want to improve your experience. Reason being, over time I've noticed that you would usually have people attack you and you'll eventually end up losing in case you don't do the jobs everyday.
However, all of this is mostly limited to lower levels when people are low on cash and on experience. Tomorrow, I'll write again to tell why the mafia wars craze is taking a low now. Till then, take care.