Because we want to allow specialization to increase efficiency. Rather than the nail company selling its nails to hardware stores and the screw company selling its screws to hardware stores, it's more efficient if they both sell to a supplier that can then sell to stores - it means only O(N) rather than O(N*M) different relationships to maintain, and the nail manufacturer can concentrate on its core competency of making nails rather than having to employ a bunch of people who know how to get their products in stores.