I think many fat people do know how to eat properly or smart, they just don’t like that food, so they eat what they want and eat as much as they want.
I know how to eat smart and I eat smarter, but I eat stuff that isn’t great for me. I don’t eat more than I should and I take a lot of supplements and herbs and vitamins to make up for some of the bad stuff.
The older I get, the more I think that there's very little that isn't great for you. About the only things really bad for you, in my opinion, are hydrogenated vegetable fats like Crisco and margarine; fake sugar (sugar substitutes); high fructose corn syrup ... and that's about it. Those things are downright bad, and I only eat them on rare social occasions.
Everything else is good for you, but of course sugar (good old cane sugar) and anything with white flour, is better used sparingly. It's all great for you. It just has to be in the right proportions.