It's largely a side-effect of the heavy marketing of processed foods post-WWII in America. Companies producing rations pivoted into producing processed food for

It's largely a side-effect of the heavy marketing of processed foods post-WWII in America. Companies producing rations pivoted into producing processed food for the consumer market, which was marketed based on its speed and simplicity. The result is that the poor are either working so much that they value the speed of processed food, or they simply don't know any better and buy the processed food rather than cooking their own more nutritious food.