They've tried. Several times, clothing designers unsuccessfully petitioned Congress for a new type of legal protection (they last tried in 2009 with a proposed three-year type of protection). One costume designer attempted to get her work through the Copyright Office by claiming it was a 'soft sculpture -- but the Copyright Office later rejected it. Although you can't protect clothing design under copyright, you can protect unique artwork featured on clothing, and you can protect arrangements of fabric -- for example, quilt designs used on clothing. So if the clothing you are copying has unique imagery, copying that would be a no-no.
What about trademark law. Some clothing designers have tried to protect clothing as a form of trade dress. That hasn't always worked so well. In a 2000 Supreme Court case, Samara Brothers, a clothing company that manufactures a line of children's one-piece outfits appliquéd with hearts, flowers and fruit, claimed that Wal-Mart copied Samara's designs and sold the knock-offs at a lower price than offered by Samara. The Supreme Court ruled that Samara's designs could not be protected under trade dress law unless Samara could prove that the designs were used to identify the source of the product (Samara) rather than the product itself. In other words, in order for trade dress to apply, a customer seeing the arrangement of appliques of hearts, flowers and fruit on children's clothing would have to think of Samara as the source in the same way that a customer who sees the Nike logo on products associates those products with Nike. In the Wal-Mart case there was no evidence that customers associated the designs strictly with Samara. We're not sure how well-known the other company's designs are but if they are well known by brand name and the aspects that you copy are non functional, you may get hassled. Similarly, the Dear Rich Staff believes you may get hassled if you copy any element over which the other company claims trademark--for example a brand name or logo.P.S. Here's a helpful article on the subject.
