Fast food in the US is significantly more expensive than preparing your own food from basic ingredients. There may be some lower-income people who eat lots of fast food, but it's certainly not because it's the economical choice.
Learning to cook economically and edibly is a barrier to entry. And there's an ongoing higher time-cost per meal due to planning meals, getting ingredients, and (finally) cooking. So it's not suuuper surprising to me that people working 2+ jobs to cover basics end up eating a lot of fast food.