Any chemical can be considered substitute of textiles. The problem is those chemical are no comfortable to the human skin. Neoprene for instance, made of Polyurethane, is the most common known best substitute of a textile. It's an excellent performer for all uses that textiles could be used. Then, why is not a substitute? Well, PET textiles have different applications not only for cloths but for unlimited uses and companies like to have massive production of PET commodity instead of focusing only in one market (expensive and cheap). In other words, PET, nylon and all synthetic fibers are more suitable for many applications that polyurethanes.

PET (polyester) is recyclable, by the way.