Because that is the position of this Cracked article. See also this SD poll here. My first reaction is that, while said thesis might contain a germ of truth, a goodly percentage of people who do take off the weight will keep it off (most of it at least). But other factors may be in play-I simply let myself go after having a normal weight for most of my adult life, and only took action once it became obvious that I would weigh 300 before too long if I didn’t do anything (got up to 243)-have gotten myself down to 186 and intend to maintain my current exercise and dietary habits (which aren’t all that stringent-20 to 30 min of hard pedaling on the exercise bike and sticking to 1600 calories or so a day).
But perhaps others who have always been overweight need to take more drastic action, their metabolisms are much slower or something and thus it is much easier to backslide and put them back on. Doctors also say that this kind of yo-yo dieting can be dangerous. Poll is coming.