There are a lot of environmental reasons why people are obese. However, it is within the grasp of everyone who wants to lose weight to do so.
Not everyone can be ripped at 7% bodyfat year round, but most people can get to normal weights, even if on the high of normal?
If I remember correctly, as a general rule, a man over 20% bf is starting to become overweight. However, obesity doesn't really kick in until the guy is 30%+ bf.
Some obese people are easily over 50%. That is nuts! that means half the weight you are carrying is just extra fat that you don't need.
If an obese person subceptible to negative external factors made a bit of effort, while it would be very hard for them to become lean, they could at least get into the 20-30% range and would make a huge difference to their health and lifestyle.
There is wide range of body types out there. According to the BMI, I'm obese, with a BMI of 31.8, yet the last time I had my body fat percentage calculated (last week, by a professional with calipers) it came in at 13.2%. In order to reach the "normal" range, I would have to lose every ounce of fat I have, plus an additional 25 pounds, and even then I would be just at the border of normal and overweight. It's not going to happen.
My point is that there's no "one size fits all" for body shape and health. I'm not a runner, but people I know who are routinely talk about seeing overweight and even obese people running marathons and passing up the skinny people. I'm becoming more and more convinced that weight by itself isn't a good predictor of health, but it is often associated with other things that are, and it just happens to be more visible.
There may be other mitigating factors, but the fact remains the vast majority of people can lose weight by changing lifestyle. If you look at an extreme case like Biggest Loser on US TV, these are very obese people who often give up and die or use shortcuts like fad diets or stomach stapling. I think even out of this group only 5% would ever have to resort to something that extreme. The show demonstrates that extreme problems can be met with normal weight lose measures, ie: the formula Galaxy uses to create a calorie deficit (albeit extreme exercise) . I like using the BMR weight loss formula, and its almost impossible not to lose weight that way. One extreme client I had lost 15 pounds in a week and a half using this method. He was a person who never had any limits to his eating whims and rose to almost 380 lbs. He lost 70lbs before some personal problems took him away from his weight loss goal. No fads needed.
I agree that fads are a bad idea. In fact, I think the whole concept of a "diet" is a bad idea because it implies that it's a temporary change until the weight is lost. If any lost weight is going to stay off, it can't be because of a "diet", it must be because of a permanent lifestyle change because as soon as the change stops, the weight is coming back.
I read something recently about why it's hard to keep weight off. Statistically, almost no one keeps it off long term. Here's an article I came across a while ago that summarizes some of the most recent research on the subject.
I haven't read it in a while, and don't have time to reread it now, but here's what I remember from it. It explains, in plain English, what science has learned about why people who lose weight can't keep the weight off. Basically, what it comes down to is that people who were once obese but have lost weight are metabolically very different from someone else of the same weight who was never obese. Hunger and satiety hormone levels are such that the formerly-obese person (FOP) is driven to eat. MRIs show an enhanced response to food in the brain of the (FOP). The cellular metabolism slows down such that the FOP needs a few hundred less calories per day. Calories are more easily stored as fat and exercise is less effective. And, these results aren't just for a short time after losing weight. It's long term--at least 10 years and probably for life. These metabolic changes kick in once a person has lost about 10% of their weight.
There's a lot more to it than just eating less and exercising more.