The "valuation" is based on market capitalization, so the market "supports" it because of the mass hysteria of "perceived value." When a big IPO gets automatically added to a major stock index (such as NASDAQ or S&P 500), all the mutual funds which include holdings that mirror the indexes will automatically buy and hold shares. Lots of employee 401k money and pension fund investments are supporting the mutual funds, which are perceived as "safe."

To put it in fewer words; the stock market is rigged.