And, what’s more, the U.S. Open is likely the most difficult tournament in the U.S., as far as the course itself. Unlike most of the other U.S. men’s tournaments, which are administered by the PGA, the U.S. Open is run by the USGA.
The USGA aims for having the best scores of that tournament be no better than about even par, and prepares the courses with particularly challenging setups (narrow fairways, very long rough, turning short par 5 holes into par 4s for the tournament, etc.). Some years, even the tournament winner shoots above par.