Are you serious? Look at that map. Those yellow areas are absolutely massive. It includes huge swathes of suburban areas, not "dense cities". I mean look at Dallas, Atlanta, or SoCal in that pic. My point is that in those suburban areas most people are not within a reasonable walking distance of a grocery store.

Just because they're centered around a major metro area doesn't mean the people living there are close to one. I live in on of those cities in what someone would consider an "urban/suburban" area. For me to walk to my closest grocery store it would take me 30 minutes each way according to Google maps. That's not reasonable. Keep in mind that you've got to actually carry the groceries on the way back. And I'm probably in a denser area than most.

EDIT: According to the USDA, americans are on average 2.2 miles from a SNAP authorized store. That's a 45 minute walk each way. No Americans are not just "lazy" for not walking to the store. It just isn't a reasonable think to do at all.