There's a difference between being fat and obese. Obesity is when the BMI of a person is at least 30. The safety area of weight is to have a BMI of 17 to 25. The amount of obese people in the US has exceeded thirty percent in the last decade. Overweightness can be attributed to different genetic factors because people may eat a little bit more or exercise a little less than what than what their body can maintain, while the next person's body may be able to handle it. Obesity is caused by eating a lot of unhealty food along with a sedentary lifestyle. It doesn't matter who it is, they will become obese, unless the person has a super fast metabolism.
EEveryone can change their weight, but the diets that people do stop working when the person gets off them, and most people don't have the desire to continue a workout after about a month of doing it. Sure, they can, but do they want to?
It's not about dieting. It's about having a healthier lifestyle, a commitment. People can't expect to stay skinny from a diet. This is why people always complain that diets don't work, and they don't. There is no point in even trying. You should change your entire diet for the rest of your life if you are going to be healthy and the right weight.
Plenty of people want to maintain a workout for over a month. It's all about commitment and if you can't stick to it you obviously don't have enough self-discipline.