1) Most people grossly underestimate how much they eat. They have no clue the calories they are eating.
2) Most people do not have a metabolic problem.
3) You just can't exercise yourself to good health. Though studies have shown that you lower your risk of heart disease if you do some moderate level of cardio every week. They studied two groups of overweight people, and those who worked out and still were overweight were more healthy. Still being active and at the proper weight helps a lot more.
4) Most people will claim to be at a health weight when they are actually overweight. Meaning they are use to seeing a culture of unhealthy people. This isn't the whole, every magazine has a sexy girl on it. Its the fact that people are just use to seeing large than normal people. This changes their perspective. My friends went over to Europe for a vacation, came back and were shocked about how fat American's are. It took a different view to get them to realize the drastic change in body size.
5) The food industry is designed to engineer food to be tasty and want us to crave them. Salty + Fatty + Carbs = addictive. Especially if you look at sugar and sweeteners which activate the same areas in the brain that cocaine does.
6) People have access to massive amounts of food. We do not have to go kill our food anymore. We don't have to worry about starvation. Meaning it is paramount that people be aware of what they are eating and why. People eat because they are emotional. They eat because it gives us pleasure to. All these things cause overeating. Basically the deck is stacked against use big time in trying to control what we eat.
As for Glutten, carbs, fat, protein. What ever, the major concern is portion control. Given all the additives and chemicals in processed foods don't show up on the scale, but I don't think they are good for the body in the long run. Who knows if all that crap is causing Alzheimer's. The food industry will claim ignorance, and the fact no study shows it. Yet, this type of damage could not have enough sample size for a few generations.