Your final line is relevant "Quality of life matters."
And for some people, "quality of life" is maximized by avoiding exercise (because they dislike it and so doing it lowers their quality of life) and instead doing computer stuff and eating tasty food. It is true that they will have health issues, but for most of their life those issues are minor inconveniences, and they are maxing out on quality.
Maybe they die at 85 from a heart attack or some other issue related to poor heath, and that's ok with them. The years they gave up were worth the enjoyment they got from the years they had.
Obviously, you have different values. Which is just as valid. We must all make that "existential trade off" for ourselves, and each person is going to wind up somewhere unique on the spectrum. Someone else's conclusions might seem stupid to you, but that is not because the conclusions are ill-informed or unwise, but because their values are different than yours.
The only stupidity is in failing to recognize that.