There's a couple of reasons that's true. SC's question assumed that an upcoming recession would be a time to sell stocks and move into a mutual fund. That's not necessarily true. In fact, a recession could be a good time to buy stocks. The reason is simple.
People buy and sell stocks based on what they think is going to happen in the future.
When most people are thinking about a recession stock prices have already been adjusted for it. So, to put it bluntly, it's too late. There's another danger for SC to consider. Trying to guess the direction of the stock market is tricky business. To do it successfully you need to buy and sell before other people do.