Because it is hard to track. For example you have say Turkish made clothes being sold into the EU. It would be extremely easy to set up a company in say Switzerland, which owns the brand rights for those clothes and makes all the money. Turkey is not part of the EU and not a tax heaven. At the same time the product is made in it and they do not have to disclose any relations of their companies.
So to do it properly, you would have to get nearly all countries in the world involved in it.