Just because there are more of the "general population" doesn't mean that the subset that are "professional athlete" isn't doing it at a higher percentage.
And there are other, non-extreme, things that also wreck the body. Working in the trades: plumbers, electricians, HVAC, construction, etc. will often leave you with bad backs and bad joints.
And yes, living your life at the extremes is going to have extreme effects on your body. Sedentary is the other extreme of professional athlete.
But even still, as things hurt more, we will be more cautious about endangering ourselves. Or even wondering if we are capable. If I can't walk without a limp, I'm sure as hell not running from danger. That's not cortisol driving our behavior, that's our behavior driving our cortisol.
Regarding your comment on cortisol. Plausibly the relationship between cortisol and behaviour here is bi-directional, as is common with hormones. High testosterone influences me to go to the gym and my workout influences my testosterone, etc.
Mind you, I wasn't completely sedentary before, but I guess that Garmin made me move about 40 per cent more.