Wow, did you major in oversimplification?
The issue, many suspect, is not just overeating and lack of exercise, but of the *kinds* of food people are eating. There was a CNN reporter who spent a month living on food stamps down in New Orleans just to see what it's like, and what he discovered is that for the poor, it is damn near impossible to maintain any semblance of healthy eating. CFIs and regional FOs subsisting on ramen can vouch for this.
But the issue, I think, revolves around the food manufacturing industry and the proliferation of high-fructose corn syrup, which is in everything from ketchup to bread. Use of high-fructose corn syrup in food manufacturing really took off in the 1970s, which, when combined with a major change in the FDA's dietary guidelines from the four food groups to the carb-heavy food pyramid, changed what people eat in an incredibly fundamental way.
There's waay more to it than diet and exercise, man. In 2000, I lost 70 lbs doing Atkins, eating bunless double quarter pounders, caesar salads and lots of eggs, while getting absolutely zero exercise. Conversely, In 2004-2006, I gained about 45 lbs despite 2 hr workouts 4 times a week, while not really watching what I ate. My mom has a thyroid disease called Grave's disease, so it's very likely I have some sort of thyroid deficiency myself, haven't had it checked to find out for sure. But I have a friend who had thyroid cancer and had her thyroid either removed surgically or destroyed by radiation therapy, who tells me that if your thyroid doesn't function properly, it doesn't matter how much you work out, you won't affect your metabolism in the slightest. Getting on the right dose of thyroid replacement drugs, and your metabolism will finally get under control.
Bottom line, when it comes to the human body, we know so little about how it really works than to make sweeping, naive statements such as those you're so fond of making.
See you in 10 or 15 years after your metabolism slows down, champ. We'll see what tune you're whistling then.