I know, because I cook/eat a lot of rice and beans. I have add a lot of fruits etc in between meals to feel good. The dirt cheap vegetarian foods people tend to suggest are not really foods you would eat long term without feeling hungry and tired a lot. With that in mind, I can see someone whose life generally sux to at least eat food that makes them less miserable.
Also, mac&cheese is quick cooking and beans are not when you cook them.
It also has other health benefits: https://academic.oup.com/ajcn/article/94/3/869/4411867
As in, it is ok lunch, but I am hunger in shorter time then with meat or say mac&cheese. Importantly, when I try to eat only that for 2-3 days, it does not work at all.
It's fresh fruit and veggies that you will tend to see the least of in c-stores.
I think a lot of it has to do with availability. People in poverty are typically less well served by large grocery stores, and are more likely to shop in convenience stores. Convenience stores are less like to stock low margin products like bulk rice and dried beans, and more likely to stock higher margin processed foods.
I'm not an expert though, could be many factors.