Exercise is anything but relaxing (except for perhaps afterwards). More likely it's mostly a status indicator. People want to be "a person who runs marathons", or be a very fit person, because in their mind it gives status.
I think you need to "relax" your definition of "relaxing"!
For somebody who crunches numbers all day, it's very "relaxing" to go out and do something that stresses another part of your being. The overworked part gets to take the time off while the underworked part gets a run-out.
That's definitely not my experience, although of course "relaxing" is subjective and can differ person to person. I think research supports that endurance exercise is good for calming and similar mental health.