A bag of spinach lasts about a week, fresh meat a few days. Meanwhile a retort pouch of chili or curry lasts forever and doesn't even need to be refridgerated. There are unprocessed foods that last almost forever like dried lentils, rice and oats but most people don't want to eat those things (or at least not for every meal).
What do poor people tend to eat? Fast food burgers and other high calorie, low cost meals that are readily available. No prep/cook time, minimal cost (at least on the surface) and hot and palatable to most.
As someone who cooks all my food from raw ingrediants, that's absolutely a luxury but it's not a money expensive one, it's a time/management/availability luxury.