How about quality of life? Those extra 25-30-50 lbs or more creates havoc on knees, mobility, breathing etc. Those extra lbs might not kill you but can make life more less enjoyable.
People make a choice when they gain the weight back. They give up quality of life to eating their favorite foods in quantity.
How do you know that? Is it not possible that the semi-starvation that they endured in order to lose weight has caused their bodies' natural defenses to push for more intake? That their increasingly intense hunger and cravings are a natural response to the low calorie diets, which the body interprets as a time of famine, such as late winter in prehistoric times? That would be a mighty powerful survival strategy for a successful species. But in our unprecedented times of plenty, it turns us into teletubbies.
That's one theory, anyway.