Visit middle America. Specifically, visit The South. You'll find that many regions in The South do no have sidewalks. If you're also only visiting major cities then you are likely not to see these differences in architecture. It is easy to form a selection bias. No one here is saying that there aren't plenty of walkable cities in America, but we are saying that you'll find a substantial overlap between non-walkable cities and obesity. If you don't visit rural America, you won't see what we're discussing. Or don't attempt it.

Since most people are familiar with California I'll provide this reference which breaks it down by county. First graph is adults, second is low income children 5-19, and third graph is low income children 2-4. You will notice how the high obesity rates highly correlate with regions that are difficult to walk around in and are more rural.

https://www.researchgate.net/figure/Percentage-of-Adults-in-...

As another analysis, Not Just Bikes talks about this topic quite a lot. Here's his episode on strodes: https://www.youtube.com/watch?v=ORzNZUeUHAM