Dieters lapse and collapse. A diet only works for as long as you are on it. Most people get bored with rigid eating plans and go off the rails from time to time. The trouble is that for many people a lapse is a sign of failure. Such people go from diet to diet hoping to find the one that will stop them from failing, but such a diet doesn't exist, and they may end up bigger than ever each time they try.
Diets fail to address the emotional aspect of overeating.
People very often eat to help deal with emotional problems such as stress, rather than because they're hunger. This is normal, but some people gain weight because they turn to food for emotional comfort or to cope with negative feelings like anger or loss. Dieting doesn't solve the problem of 'emotional' eating. If anything, it makes people more depressed - and losing weight will often make things worse, as people have to cope with the pressures and expectations that their new body shape can entail. Dieters usually fail to change their core habits.