McDonald's sells cheap food. This is the reason why, in America, "fresh food" is expensive.
Okay, as a poor person myself, I am incredibly tired of this argument. After my other expenses, I have less than $40 a month left over for food, and so I have to pay very close attention to the price and nutritional content of my food. And the thing is, buying a mixture of raw ingredients(mostly flour, sugar, and eggs) and fresh seasonal produce(produce, meaning fruits and veggies. Any meat is expensive, and is well outside of my price range most of the time) is far cheaper than getting that non-nutritional garbage from fast food joints(believe me, I have looked, and if fast food was cheaper per nutrient, that's the only thing I would be eating because every dollar I can save really does matter in my situation).
Of course raw ingredients and fresh produce do come with their own particular downside; it takes time and effort to prepare it, and it also takes time and effort to learn how to cook. Some poor people work multiple jobs or long hours, and when they come home at the end of the day, they're too tired to put in the required effort, or they don't have enough time left in the day to cook a proper meal. It's faster and easier to either buy some fast food, or to just heat up a frozen microwave dinner, despite it costing more. Maybe they value their free time more than they value the monetary savings. Or maybe they can't do math. Or maybe they're just lazy. Most likely, it's a combination of all of the above. But I don't know about other people, all I know is that when I come home after working for 10 or 12 hours, I'm really tired and don't want to put in the time and effort to cook a meal despite being quite hungry, but I cook anyway, because that's the sound financial decision.