In my neighborhood, a wealthy liberal enclave called Marin County, preparing something paletable from raw ingredients is usually more expensive than cheap salty starchy food.
There are no cheap cuts of meat at my Safeway.
Most people are horrid cooks. They might think they can cook, but usually it's just awful. (Learning how to cook is not my point though.)
Poor people have huge problems, and cooking a nutritious meal is down on the priority scale.
Plus---for many poor people, the little bit of enjoyment is eating something that tastes good. I can see why the poor gravitate towards the lousy quick food.
(My all means I am poor. I don't eat crappy food though. I was a cook in my teens though. I'm glad I learned how to cook, but understand those that didn't.)