This article asks a good question.
But it’s an obvious answer. the supply chain is like a cooperative, and all the participants have to share in the benefit or they will look elsewhere. This accounts for the rise of 3PL’s. A 3PL has a better chance of acting like a ‘supply chain manager’ by balancing the needs of each participant. And it is so complex in a long supply chain that special attention and expertise is demanded. A specialist may be able to lower the transaction costs of planning, monitoring, and controlling for the members.