I think it's because some people need a challenge, especially a physical one. To anyone who works a job which doesn't involve physical labor, doing something like this can often be the ultimate relaxation in a balancing sense because it's opposite of what they spend 40+ hours per week doing. And that doesn't count the many hours sitting motionless in a vehicle or train.
For some of these people, mindlessly running laps around a track or endless repetitions with weights is only a small step up and not worth the effort in mental rewards.
Btw, Re: my above posts, I've only hit a low of 9F backpacking so far.