If September is historically a bad month for stocks, is there anything stopping people from selling their stocks just for September?

I haven't paid attention to economics until very recently, so apologies in advance if this is a fairly simple question.

As someone who just started learning about investing ands stocks, I came across many people saying that “September is the worst month for stocks”. So this got me wondering: if we know that September has had a bad history for a long time, what stops people from just selling all(or at least most of) their stocks for the month and buying them back towards the end of the month?

The possibilities that I thought of were:

1. It doesn't matter for long term investors (wouldn't it be better to keep your number as it is than to have it drop a few %?)
2. The past is not an indicator of the future (even though that does seem to be a strong trend?)
3. You may miss out on a possible positive return (I think this relates to the issue of “trying to time the market” which I would find valid for long term investors.)

Are there any other reasons for this, such as taxes or other complicated issues I am not aware of?

Edit: I didn't expect so many responses, but I read all of them and they were very helpful in answering my curiosity. It was interesting to learn a little bit more about investing, and it seems like there's still so much more for me to learn. Thank you all for the kind responses!