Our bodies each have an equilibrium of calories consumed/burned that it really wants to be maintained. If you start dieting to reduce calories consumed, then it automatically adjusts so that your body burns fewer calories just for existing, thereby reducing the deficit you created with your diet. So people see the diet not working and give up rather than taking the time to dial in a sustainable dietary calorie deficit that allows for weight loss without leaving you physically and mentally exhausted by your bodies compensatory reaction.