The excess calorie consumption that has contributed to the rise in obesity directly correlates to the rise in consumption of poor quality foods like junk food and fast food containing sugars and other refined carbs, and fast foods. Low-carb advocates argue its the bad carbs while others say its the fats, but in truth consumption of both refined carbs and oils has gone up, so that argument misses the point. Unfortunately, the US government contributes to the situation by subsidizing commodity crops that are used to make sweeteners and junk food, or to feed to livestock. This is due to an obsolete farm bill dating back to the great depression which was originally intended to keep small farms going. A similar situation existed for decades subsidizing tobacco growing, again based on an obsolete law from same era. I assume these laws stay on the books due to lobbying efforts on behalf of those benefiting from them. I’m not a fan of misusing lobbying in that way.
One explanation why calorie-rich foods like overly-processed junk food contributes to weight gain is that they confuse the body’s “set point” mechanism for maintaining weight. That seems plausible to me. I don’t know about you, but for me it always seems that mechanism works a lot better in one direction to the other. It is ever-vigilant when I’m trying to lose weight but seems rather lackadaisical when I’m gaining. As discussed in the video in the link, it does a much better job with food that is in a more natural state than with overly processed food.