Because yours cost $60.
People have been practicing yoga for thousands of years. Many, on nothing but a slab of stone and dirt. Do we really need to spend a fortune on customized, brightly colored materials to stretch out bodies? Obvious answer alert: NO.
In fact, your own home is equipped with a bunch of items that are just as useful in your "practice" as a Gaiam-certified block. You may have to overlook a sports team logo or a little cat hair, but it's a small price to pay for not paying a large price.