Most diets do actually work, that is; while you stick to that diet. However, it takes willpower. As soon as the willpower weakens, people give up on their diet.
Unfortunately, once a person starts to diet their body automatically lowers their metabolism. The longer that person stays on the diet the lower their metabolism will get.
After the diet, even when a person starts to eat normal amounts of food, they gain weight.
This continues until they are heavier than they were before they started the diet! Instead of losing weight – they end up heavier than before!