There are some cases of "food deserts" where the only store for miles is a convenience store (which generally has foods with high calories and low nutritional value) so I can see how someone who doesn't have a means of transportation can be overweight. But not full on obese. And even then it has to do with education as well, not just availability of certain foods. Everyone just likes to use someone's rare specific case that has multiple factors playing into it as an excuse for them being fucking fat.