It's from a dramatic rise in inactive lifestyles by younger people and a lot of sitting, with the predictable consequences. Behavior which would have been far less common before mass adoption of video games and personal computing in the home, and then everybody sitting/laying around their homes with their smartphones. Young people go out less, they have a lot less sex, they're far less social in-person. It all goes together.

These people under 50 were the first generation to sit around at home for long periods of time playing video games. The NES came out in 1985-86 in the US and home video gaming has been mass adopted since. From there add on the full adoption of sitting down at a computer for work in offices across the 1980s and 1990s. Then throw in the full adoption of home computing in the 1990s with Win3 & Win95 + GUI. And for the past 20 years, throw in the smartphone.

It's the sitting primarily, hours and hours of it every day.

I don’t drink, I never smoked, I eat clean, I run, I swim, I go to the gym. But I’m also at my desk for 13 hours a day on average and I think it will kill me one day. Trying to stand as much as possible but it’s not always possible,

> It's the sitting primarily, hours and hours of it every day

...do you have a source for this causative link? It would seem like it's highly testable.