Most food deserts are found in urban areas, where most people live.

The size of the US is very important when discussing food deserts, including within urban limits. Many cities are not friendly to pedestrians and are set up so you have to drive to the commercial centers to get food. When you couple that with poor public transportation, it doesn't make it easy to get fresh food. Mixed use areas are also not as common, since residential and commercial zoning keeps stores out of residential areas.

Like, I have something like 8 grocery stores within 10 miles of me, but there's almost no sidewalks in either direction, and in one direction, you're walking on highway to get there, which isn't safe.

Also, a four hour roundtrip to a grocery store isn't really feasible when you also have work, kids, chores, etc.

Looking at the London metro area, Google tells me that it's about 600 square miles with a population of 9 million. For comparison, Detroit Metro area is almost 1.2K square miles with a population of 3.7 million. Houston is 10K square miles with a population of 7.1 million

So yeah, the size of the US is definitely a factor in food deserts. Outside of maybe NYC metro area (4.6k square miles with 20.1 million people), we just don't have the population density in most of our urban areas.