Then, why do people bike? I go out on the mountain biking trails to give myself a challenge. I love the adventure. Clearing a log jump is no small feat, getting the courage to do so is even more of a challenge. Conquering an extremely steep or an unusually large hill is something to be proud of. It is something that is achieved over time, and is seldom something that comes without practice and training.
I also mountain bike for the fun. My friends and family do the same and it makes for a fun summer activity. Its great to explore new trails with people you know. Some of my most fun experiences last summer were on backwoods trails with some of my family members.
Finally, there is no disputing that mountain biking gives you quite the workout. I love the hills. In fact, without the constant hill climbs, it would not be much of a course. The constant climbs build your leg muscles and your cardiovascular strength at the same time.