There are diminishing returns for cleaner water past some point, as you say. Cheap and unhealthy foods is a well known major reason, but there are other major factors too.
American cities place low priority on walkability: it's usually cheaper for the real estate industry to build out rather than build up. Regardless of if that's intentional urban planning or not, it reduces the minimum amount of exercise people get.
Another major reason is infrastructure maintenance. I'll bet the recent leaded water issues in many cities made a non trivial dent in America's longevity stats.
Finally, my personal take is that this may be a side effect of some elements of American society. Maybe it's a combination of it being such a large and diverse country. I grew up in America, but I've been living in smaller, more homogenous Asian societies the last few years. Homogenous demographics have their problems, but it seems like they're generally less willing to screw over their fellow country people for a quick buck. (China is worse than America in this regard, so I added a large population to my hypothesis.)