Pretty much every neighborhood created between 1940 and 1990 has no sidewalks, and even after that the zoning is such that the only place you can walk is to other residences.
I don't think it's carbs as such necessarily, but the glycemic index of the foods and their addictive qualities - i.e. how easy it is to way overdo the glucose spiking.
Yes, if you cut the carbs the problem is solved, but there are different kinds of carbs and some kinds in moderation can be eaten regularly without causing big glucose spikes and metabolic disease.
Personally I go for low carb to make it easier, but I still eat limited fruit and starchy vegetables and rice and beans. (Some kinds of fruit do spike glucose and I need to be careful with.)
There's truth here, and, as I understand it, carbs with high glycemic index do have the effect of blood glucose and then insulin spikes which cause metabolic disease and lead to diabetes which is the primary thing that's deadly here.
I don't think it's just the carbs, but they likely don't help in today's society. We became too efficient at packing carbs in our diet and at the same time too sedentary.
Beyond macro-nutrients, what I think the true reason food in the US is bad is because of the other stuff. Many things banned in Europe (which is special because they use science for improving quality of life before profits) are not banned in the US, even though it's an advanced enough nation to be able to deal with the ban and look for better options.
The problem in the US is that profits are of utmost importance. Anything that lowers your costs is good for business as long as it's hard to point at it (does colourant X cause cancer? maybe we need more studies, or just didn't know, we'll try better next time).
Japan, Vietnam, France, Italy, etc. would beg to differ. But they also don’t spray their wheat with roundup so it’s probably better to stay away from carbs in the US just on that basis.
Also, with a pinch of pedestrian-hostile design, where some places don't even have sidewalks.