It's just part of the illegal nature of the cocaine business, everybody wants to make a little bit more money, so they cut the product a little bit more. Of course, as StaySedated said, that can vary considerably. Some places have better cocaine than others, others always get "pure" cocaine (cities that are closer to distribution points, etc).
I'd imagine that 100% pure cocaine is available right when the cocaine is shipped from its source country into Mexico and then into the United States, but the Mexican cartels probably step on the product even at their level...just speculation.