The obesity trend has happened almost in lockstep with the proliferation of highly processed foods. Butter and animal fats being replaced with low quality, hydrogenated vegetable oils. Cane sugar being replaced with high fructose corn syrup and other highly processed sweeteners. Sodas and sugary juices replacing water. Food like substances with little to no nutritional value designed solely for taste and texture.
These things are calorically dense while containing nothing the body needs to thrive (though the calories will allow it to survive). They are easy to eat in large amounts and leave you feeling hungry. And unfortunately, these are the most affordable and readily available foods in the United States.
I don't think this is a conspiracy. It's just capitalism. These low quality ingredients are cheap and extremely shelf stable. In addition, the government subsidizes the production of this garbage.
So to say obesity has persisted through everything we've tried is a bit backwards. It would be more accurate to say "a percentage of the population has managed to avoid obesity despite all of the things we've tried."
- Make healthier food options more affordable and readily available - Better nutrition education - And if you really want to get the government involved, ban the use of some additives, oils, sweeteners, and dyes that allow the creation of many of these highly processed foods