I don't want this discussion to turn into an attack on you, so let's get one thing straight. You are absolutely right that there are things like grains, beans, and pasta that are cheaper than junk food like potato chips, and that milk is cheaper than soda much of the time.
However, what many people fail to consider are the associated costs that go along with preparing these sorts of meals that we take for granted. We take functional kitchens for granted. We take pots, pans, ovens, refrigerators and time for granted. A Happy Meal is a Happy Meal and you buy it and you eat it. The costs stop there. When you buy ingredients at the store, you pay for them and you pay for the electricity to cook them and the water to clean things up. To most people on here, this probably sounds like a laughable argument, but for low-income families it's a major factor.