If September is historically the worst month for the stock market, what would prevent people from selling stocks in September?

As someone who has just started learning about investing and stocks, I often hear that "September is the worst month in the stock market."

If people know that this has been deteriorating over a long period, why do they sell all (or at least the majority) of their stocks in this month and not buy again at the end of the month?

The possibilities I have come up with are as follows:

1. It may not be important for long-term investors (holding on may be better than a few percent drop?).

2. The past cannot predict the future (although it appears to be a strong trend).

3. There is a possibility of missing out on potential profits (I think this is related to timing the market, but it is effective for long-term investors).