A lot of people live in so-called "food deserts" [1]. That means they don't have access to fresh produce or bulk ingredients like milk, flour, eggs, beans, rice, pasta, or potatoes. Instead, they're stuck with convenience store food which is mostly junk food.
If they wanted to buy healthier food they'd have to travel a long way to reach a grocery store, a problem greatly exacerbated for those who do not own a car. It's not easy to carry home 10lbs of potatoes and 10lbs of beans by public transit, let alone on foot.
[1] https://en.wikipedia.org/wiki/Food_desert