http://americannutritionassociation.org/newsletter/usda-defines-food-deserts "food deserts are often short on whole food providers, especially fresh fruits and vegetables" Not that it affects a large majority of people, but there is a small population of people in the US who literally do not have access to healthy foods like you talk about. For example the only nearby place to buy food would be a gas station or convenience store, otherwise the closest supermarket is 1-10mi+ away. This is the group of people that the professor is addressing when he says its "unrealistic" to get people to stop eating these foods entirely.