I'm a great fan of my daily walk. So not saying this is necessarily wrong.
But there's no theory! e.g. mental health isn't understood, so whether exercise is of lasting benefit in that department is unclear. Consider that people pay good money to sit for a week or more to try to elevate their well-being. Consider that athletes have their fair share of mental health issues, and that even top athletes live less than 3 years longer than average. (Which seems like a small difference.)
Consider that people who move about constantly will eventually drop dead of heart failure:
https://en.wikipedia.org/wiki/Dancing_Plague_of_1518
Examples such as this are perhaps misleading edge cases. But theory would give some insight into why they are misleading. At present we are blind.