When an investor focuses on short-term investments, he or she is observing the variability of the portfolio, not the returns - in short, being fooled by randomness.
History doesn't crawl; it leaps.
If you roll dice, you know that the odds are one in six that the dice will come up on a particular side. So you can calculate the risk. But, in the stock market, such computations are bull - you don't even know how many sides the dice have!
Life is a tightrope between two errors: generalizing the wrong particular and particularizing the wrong general.