Because it shows that under various economic conditions, the market value cycle worked. In the past 120 years we have had inflation, depression and wars, and the value cycle got investors out of over-valued markets and into undervalued markets. Earnings would have to triple and the market stay flat for that to happen. Greed and Fear . Those are the two most important variables in the market equation.
Fear is what makes investors sell at the bottom.
A rational investment strategy would have you buy when stocks are cheap and sell when stocks are expensive. Instead, investors do it the other way around. The chart to the right shows investors sold mutual funds for a year after the crash in 1987, when they should have been buying. If you haven't done it yet, you need to re-position your investments to take advantage of the next investment cycle, and not depend on the last one going on forever. Whether you like it or not, the markets and economy are both cyclical.