I work in healthcare and this is my understanding from the responses of patients that I encounter - fast food is cheap. One of the families I work with, both parents are diabetic and the children are all overweight. I asked them about their eating habit and the answer was : McDonalds for breakfast, lunch, and dinner. Why? Because it's cheap to eat at McDonalds for a family of 4 than it is to buy vegetables at the market....for 3 bucks, you get a whole meal for one person. $3 at Safeway, you only get..a cabbage and some tomatoes?
They didn't understand that their eating habit is going to cost them much more in health expenses