It is worth reading Mackay's "Memoirs of Extraordinary Popular Delusions and the Madness of Crowds", a book written in 1848 and a classic reminder of how crowd psychology easily leads to irrational behaviour. The problem is that the irrational behaviour is only evident after disaster has struck.

What keeps people from cashing at least some of their money out, is simple greed. The fear that someone will benefit from your sale, even though you have already made a substantial profit. No market can continue a constant upward trend, the trick is to cash out in trenches ensuring that if the worst happens you do not lose everything. By all means keep some investment in the market, but keep it proportional to the evident high level of risk.