Obesity is a complex problem. Junk food and lack of exercise may be contribute to it but contrary to popular belief dietary restriction is ineffective.
I think you've fallen into the same trap as many other people have.
I used to share the same view as you but I've realised I was wrong but I don't expect I'll be able to convince you, as I doubt arguing with someone on the Internet would've changed my mind either.
Calorie restriction does solve the problem. People who say diets don't work are the people who define a diet as some kind of temporary action. There's no reason for it to be temporary.
It isn't losing the weight but keeping it off which is the problem. After an obese person losses weight, their body fights it. They experience intense cravings for high calorie food, tiredness (therefore reduced physical activity) and eventual weight regain,
For most people (*) it's a question of will power plain and simple.
*) This qualification is to deal with the unlucky souls who have genuine medical conditions.
No, calorie restriction does not solve the problem of obesity. This has been known about for a long time, 60 years or more and it isn't a simple case of will power either. For some reason, doctors haven't been told about this. They keep telling their patients to restrict their diet but it's ineffective over the long term - the body fights back making restriction unsustainable.
There's a reason why weight regain occurs after a period of calorific restriction and it has nothing to do with greed or falling back into old habits but the body's starvation response.
In fact calorie restriction makes the problem of obesity worse in most cases, not better. It inflicts more stress on the metabolism and carries an increased risk of further weight gain, rather than loss.
A classic study is the Minnesota starvation experiment. During the war, in a bid do find out how to re-feed famine victims and prisoners, some previously healthy conscientious objectors were put on a low calorie diet and exercise regime until they lost 25% of their original body weight. Not only did the diet cause them to suffer both physically and mentally but when they tried to re-feed, they ended up binge eating, until they overshot their pre-diet weights considerably. Fortunately after a couple of years of not restricting, they returned to their pre-starvation weights, those who were fat before the diet remained fat and those who were thin reverted to being thin. In the long the period of restriction made no difference whatsoever.
http://jn.nutrition.org/content/135/6/1347.fullIt turns out that doing the same kind of thing to obese people has similar effects. The main difference is they generally can lose more weight before this happens and their fat does protect their body's protein to some degree but they still end up getting fat again after the period of restriction ceases.
Another interesting experiment is the Vermont prison overfeeding study where a group of prisoners who'd never been overweight before were put on a high calorie diet, in order to increase their body weight by 15% to 25%. Surprisingly many struggled to achieve this, even gouging themselves on between 8000kCal to 10,000kCal per day.
http://idealbodyweights.blogspot.co.uk/2009/08/vermont-prison-overfeeding-study.htmlHere's another fascinating article on obesity and the human metabolism.
http://www.dana.org/Cerebrum/Default.aspx?id=39307