For me, I find it the most enjoyable way to exercise. There are few physical activities that you can do in a 10 hour day and still enjoy . Mountaineering is one of those few. I can't see anyone, for example, working out in a gym for 10 hours straight to be enjoyable.
Mountaineering and biking to work provides the bulk of the exercise I get since I have a desk job and aren't in the best shape.