Same reason why Gigabyte, Asus, MSI, XFX, etc don't design GPUs of their own.
I'd imagine it's not a worthy investment to put money into the creation of manufacturing plants just to produce cards which might as well be done by companies who already have capital invested in manufacturing of PCBs and other integrated circuits. AMD and nVidia DO produce a reference specification, however, but like I stated previously, they don't produce the cards because it isn't worth it from a financial point of view. In other words, AMD and nVidia specialize in the design while your third-party companies specialize in the manufacturing. It's more efficient to specialize so they don't dabble in each others field of expertise.