Why are U.S. healthcare costs so high?

I was just reading a Seeking Alpha article about the pending healthcare deal between Berkshire-Hathaway, Amazon, and JPMorgan/Chase, and someone who added a comment referred to this image about the rising costs of healthcare in the United States, and how those costs correlate not to an increase in physicians, but to an increase in administrative overhead. (The image comes from this IBJ.com page.)