Because I enjoy it. Really, what's not to enjoy about a strong afternoon swim? Or how about getting into the gym and ripping some heavy clean and jerks!
What about bombing up and down a remote backcountry hill just to breath hard and sweat a little? Man! I do this shit for fun. Getting out there and tearing up some country is what I call recreation. I'd just as soon jump in my kayak and paddle out a few miles vs. anything else. Movement is the lube of life. Get some sweet gear and earn some time away from grind of financial procurment and use your body. You'll be old soon enough.