Well, more than 50% of the companies have negative returns in the public market. So why do people invest in the public markets? For LPs, investing in VC firms is just to way to diversify and invest in private markets.
Far from it the median company on the stock-market has positive returns just about every way you slice it. That's not beating the market, but the median VC firm does not just fail to keep up with the stock market, they also flat out lose money.
Are you sure? Net market growth is positive since 2009, so that would mean that the growth would have to be limited to an increasingly smaller number of companies.