Traditionally 'poor' food is fattening because the calories were needed cheaply. Potatoes, bread, dripping, ghee, rice, cheese all bulky stuff that would keep you going.
It's a good thing that civilisation has developed and envolved to a point where people have enough to eat and generally three meals a day is seen as a good thing. Relatively speaking though the pricing is still the same hence cheap is usually fattening given we all do much less physically than we used to - but that's a digression... As is that culturally we tend to learn food from our families and not everyone has the inclination, talent or palate to learn different ways to provide themselves with nutrition.
Evolution is a slow process and denying it doesn't help any one neither does the refusal to acknowledge that cheap, quick, easy, filling food has far more selling points than a homemade ratatouille and wholemeal loaf that made badly is going to be rank and leave a person hungry.
Smugness and scorn has never been a great marketing tool to change things for the better.