When patients aren't also the payer, incentives to innovate and serve the patient are skewed, and you end up with inefficiencies and bloat.

The 2 largest industries where the customer is not the payer:
Education
Health care

Any similarities between these?

Single payer makes this problem worse. It would grant monopsony bargaining power, but the customer is then the gov't and not the patient. Hospital management then becomes satisfying their real customer, the gov't, by streamlining regulation compliance, only caring for the actual patient to the extent that is required by regulation. No benefits to innovation, to differentiating customer (patient) service, etc.