Diet drinks are not weight loss in a can. They are loaded with artificial sweeteners, which some studies have shown have similar metabolic effects on the body as sugar.
According to a new study, people who drink diet sodas might also eat more junk food than people who opt for sugary drinks or alcohol. Diet beverage drinkers might compensate by eating unhealthy foods.
While it's not clear why that is, this article in the Washington Post talks about some of the possible reasons.