Why? Because the WHO set the methodology.
The rankings in their 2000 report (they've done no rankings since) are based on an index of five factors:
Health (50%): disability-adjusted life expectancy
- Overall or average: 25%
- Distribution or equality: 25%
Responsiveness (25%): speed of service, protection of privacy, and quality of amenities
- Overall or average: 12.5%
- Distribution or equality: 12.5%
Fair financial contribution: 25%
Notice that 62.5 percent of the "grade" measures the "distribution," "equality," and "fairness" of the system. In short, how socialized it is.
On overall or average responsiveness (12.5 percent of the grade), the U.S. came in as number 1. On those "fairness" and "equality" issues, it was way down the list.
It's not surprising that socialist systems would pull a higher score no matter how good the actual health care was.
Using this famous study to justify socialist medicine is a circular argument.