Index funds have an >90% chance of outperforming managers who are paid to beat the index. Who wants to pay for something that does worse than a less expensive product? Not this guy.
A successful investing portfolio adapts to market turmoil. When one part of your portfolio zigs, you want the other to hopefully zag. This happens through diversification, which is one of the key ingredients to successful investing.
You know how often I make changes to my portfolio? Once per year at the end of July. Why? Because I know that my index fund portfolio is going to capture the market average.