Why is the S&P 500 considered "The Market"?

[Shoe]

S&P 500 is considered a good benchmark because historically it has beaten over 75% of the other funds out there. You can always find funds that beat it over the short-term, but it is hard to find funds that consistently beat it over the long-term.

A well-educated investor can probably beat it, but for the average joe, it the S&P 500 fund is a great investment.