The other aspect is in general people who are strapped for cash also tend to be strapped for time, and the vast majority of junk food and quick premade meals do

The other aspect is in general people who are strapped for cash also tend to be strapped for time, and the vast majority of junk food and quick premade meals don't have nearly enough protein in them for all the calories they contain. Very easy to end up overloading on low quality carbs and fats while skimping on protein and healthier sources of the former. As a rule the healthier stuff tends to be more expensive and less easily accessible to the busy/poor/stressed consumer (since a proper diet almost always includes a lot of cooking from scratch).