Even when it's not the insurer, it's at least a hospital. Many a doctor around me that used to have a private practice sold to one of the hospital chains, as they promised more money than by owning, solely due to superior collective action advantages. A large insurer can bully a private practice into cutting costs, but a hospital network that handles 40% of ERs in the metro area? The insurance company can lose. So everyone makes more money but the people paying insurance.