Clean soil is exposed to ultraviolet light, oxygen and rain keeping it clean. It can be contaminated by man's activities, which is part of the attraction of going out there beyond the reach of people and their dirt. Human settlements are the places with the dirt.
edit- It has occurred to me that many people can't get over the hurdle of "dirt" which is what stops them from spending more time in the outdoors. I was trying to arrange a big deal trip with an old friend from high school, I think it was a Grand Cyn raft trip. He stopped me in mid-sentence and said "I have to have my shower every morning." Think of the self-imposed limitations of that idea. Its like owning horses, you have to be home twice a day unless you take them with you. Many "horse people" spend most of their lives taking day trips because they have to be home to feed.