This is exactly the reason. If you put Buffett's holdings in a mutual fund format he will consistently under-perform the S&P. He will go down as the greatest investor of all time but his legacy should be the greatest businessman of all time. He isn't much better at investing than any other investment manager managing a mutual fund who happens to do fairly well.
Quote from Baron:
Mainly because investment professionals buy stocks, but Buffet buys businesses. Big difference.