Many Americans live outside of walking or bicycling distance to supermarkets. Many of them are poor people and live in rural, urban, and suburban areas that are known as food deserts. As an extreme example, until a few years ago, there were literally NO reasonably priced full-service supermarkets within the city limits of Detroit--a city (at the time) of 750,000 people that is more than 10 miles across in every direction (talk about sprawl!).
Food deserts mostly affect low income people, who are both more likely to live far from markets that sell nutritious food, and more likely to be carfre, meaning it is more difficult to travel to distant food sources. Public health organizations have linked food deserts to obesity, diabetes, and other chronic diseases linked to poor nutrition.
"Food deserts are defined as urban neighborhoods and rural towns without ready access to fresh, healthy, and affordable food. Instead of supermarkets and grocery stores, these communities may have no food access or are served only by fast food restaurants and convenience stores that offer few healthy, affordable food options. The lack of access contributes to a poor diet and can lead to higher levels of obesity and other diet-related diseases, such as diabetes and heart disease."
--from US Dept. of Agriculture
Agricultural Marketing Service - Creating Access to Healthy, Affordable Food
A Look Inside Food Deserts | Features | CDC
Food Deserts - Inter-Faith Food Shuttle