A quote from The Four Pillars of Investing:

"Of the four key areas of investment knowledge — theory, history, psychology, and investment industry practices — the lack of historical knowledge is the one that causes the most damage. In finance, there is no controversy: the same speculative follies play out with almost clock-like regularity about once a generation."

A whole lot of investors have come to believe that history is obsolete, and later regretted it.