My 2c is that one should simply look at the diet of lifestyle of the fat people they know, and the obesity problem will become clear.

Tons of soda. Products like Hamburger Helper instead of actually making pasta and sausage yourself. Extremely high volume of food in general. Constant, constant snacking. No exercise. So a massive amount of low quality food and nothing to burn it. Cancer, heart disease, joint pain, diabetes, heartburn, everything else.

Occasionally you run into some outré example like a family I know who eats exclusively home-cooked meals with grass-fed beef and venison yet are all overweight, but I've had lunch with them and that was some of the richest, tastiest food in my life plus all the trimmings. Also, they don't really care about exercising, or for that matter their weight.

I don't think this is complicated. Weight gain is simply calories in and calories out. The only contentious part is the psychological compulsions that cause people to eat amounts of food that they know are bad for them, sometimes while maintaining a sedentary lifestyle, which they also know is bad for them.