I think you misinterpret what they are saying. What really happens is that even though it was a good move this time, the next time you have the same feeling and make the same move, what you move it into may go down and what you move it out of goes up. Then when you buy back into stocks you are buying at a higher price than when you pulled out.

And this second scenario happens more often to people than the first one (according to common wisdom).