To say they're complementary doesn't mean that they depend on each other to an equal degree.

I don't believe it would be easy to find a use for all that cow skin. Think of the enormous amounts. But even if they did find a use for it that doesn't cost them, it's not likely to be nearly as profitable as selling it as an input to the leather industry. (If there was a more profitable way to use cow skin we'd probably be doing that by now)