It's, in part, the American diet. Americans eat an insane amount of beef, fat, grease, etc which accumulates into old age. Then older Americans see a reduction in "usable years" because basic things become difficult.
A lot of older Americans are "skinny fat". In that they might not appear obese, but their actual health compares to an obese sedentary person.
Add in the car centric culture, the obsession with luxury goods(beef is a luxury food), and the glorification of sedentary lifestyles, and you get olds who can barely walk for the last 20 years of their life.
We're also seeing a crazy uptick in people in their 20s coming down with heart diseases. I havent looked into that specific issue in a while but the leading theories were over consumption of caffeine and beef.
I grew up in rural America. I watched so many older people I knew come down with diabetes and heart issues in their 40s and early 50s. It meant they couldn't do much physically. A lot of older Americans, especially rural, spend their last 20 years trapped in their home because going outside is too much effort. In hindsight, and the data backs this up, it's clearly their diet. Americans glorify overconsumption of horrible foods. We made beef and meat a luxury good and encouraged it's over consumption by making it a "thing" successful people consumed every night.