regaining weight

I think it is because they have restricted their diet in the first place to lose the weight then go back to their old eating habits after they have lost the weight.

Diets pretty much set you up for gaining weight in the long term. when you restrict your food intake, your metabolism slows down, and your body starts to go into 'starvation mode', storing everything it can get. So you have to restrict your calories more and more and more to continue losing weight. And when eventually you stop the diet, you will put that weight back on again, regardless of whether you exercise or not, generally.

I think the main reason is because they don't change their lifestyle and the way they eat food. A diet works but if you go off of it nine out of ten times you will gain the weight back. You really need to make a whole lifestyle change.