Beans and all that isn't cheap overall: it takes will, effort and time to figure out what to buy and how to cook it. If that was packaged into cheap ready to eat burgers, spiced with something to make it more tolerable, then poor folks would switch to it. This approach isn't economically sound, though: it has 0 profit margins and it cuts deep into the fastfood & healthcare owners.

They are cheap if bought in bulk (which many poor people can't do because of upfront cost or lack of freezer space) and if there actually is a place to buy at least decent-quality vegs.

Check out https://en.wikipedia.org/wiki/Food_desert

Quality vegetables aren't cheap. At least not for the consumer.