Rice and beans are a complete diet. Potatoes and milk are a complete diet. Frozen vegetables fried with spices are tasty, nutritious and very fast to prepare. Porridge, aka oats with milk or water is also dirt cheap and nutritious.

I spent an unfortunate length of time watching my pennies when it came to my foodbudget, in a western country. Being poor sucks but healthy food isn't expensive unless you mean prepared, ready to eat expensive food.

Will power consumes mental energy. Fighting the urge not to consume the cheap sugary food all around you is not something many people have the resources to do - especially the poor, who are under stress and therefore have fewer cognitive resources to devote to willpower. Drop the politics and think about things logically - then reintroduce your emotion. If you manage it you will be outraged at the disgusting setup of modern society.

If one is very poor, one cannot afford money to get prepared food and forced to prepare food themselves no matter how weak is the will power. And that will be much healthier. The problem is that in Westen societies such extreme poverty is rare and people can afford junk food and advertisements heavily inclines them to get it.

The problem is also that it is easy to buy some cheap bad food while not saving up for rent/car/school payment. In other words, even though we can't afford our lifestyle, food dolllars are the first dollars, and we go into debt for the rest. So we can over spend on convenient food while still being broke.