It's likely not entirely due to the shareholder/stakeholder model, but the supply chain of U.S. automotive manufacturers is not a harmoneous and collaborative enterprise. While relationships between the Detroit3 and its supplier partners have been improving lately, that's only after nearlty two-decades of brutal, draconian business tactics have been employed by the auto manufactures to squeeze the maximum expense out its supply chain. This includes required supplier pricing reductions to stay in the running for future business wins, or to maintain existing business. Automakers have been known to simply pull a contract out of a supplier ACTIVELY making parts, and move it to a competitor if the automaker customer feels it to be neccessary. These tactics are done in the name of cost and maintaining car assembly schedules, which in turn maintains shareholder value.

Bottom line is, the auto suppliers are the ones that get whacked by the automakers' "big stick." Automakers call the shots, and if suppliers want to make money, they had better get on board. Or else.

So, to your second question, the answer, at least in the case of domestic automaker - supplier relations,  appears to be yes. They may say otherwise, and like I said before, things are moving in that direction, but it's a long way from "wholehearted participation in a supply chain driven by visions of collaboration, mutual benefit, and sustainable end-to-end performance for ultimate customers"