Why do people who invest in the stock market, ie 401K's have the mantra of let it ride?

A good economists this morning said that is the most ignorant thing you can do, and for that reason he was pulling out of the markets as he forsees a MASSIVE collapse. I don't understand the thinking of most, nor the host of the show, as they were truly perplexed, since everyone is being told to let it ride through the ups and downs, and god willing, you will have money when you retire. I think most just pump money in without any real oversight or direction.