Mainly because they are made from man-made fibers. In fact, more than 50% of the materials used for clothing are fibers from processes that contain plastics and other petroleum derivatives, caustic soda, and other toxic chemicals. Even in the case of natural fibers, when cultivated in a conventional way (i.e. with a chemical bath, pesticides, etc.) the result is a highly harmful fabric that is placed next to our skin. Each time that we wear this fabric, those harmful toxins will be absorbed into our skin.