Your body wasn't designed to do anything. It's the result of random mutations that survived better than other random mutations.
> The very idea of needing to spend free time exercising just to keep your body from falling apart is, well, preposterous.
A somewhat sedentary job with the option to take breaks to move around and exercise outside of that is still vastly preferable to any other combination. Non-sedentary jobs and manual labor will break and wear down your body in different ways, they aren't some ideal to aspire to.