Stock values go up when people buying stocks expect them to go up.

Stock values go down when people holding stocks expect them to go down.

Why would you hold a stock if you think it should go down? If you think the stock is valuable but in the near term should go down, why not sell and then buy in increments as it goes down?

Because not everyone is in it for the short term.

Tax reasons might be one as well, long term capital gains are taxed less.

There are few investors that can spend the time it takes to be active like that.

Most people buying individual stocks are better of buying ETFs anyway.

In the end it's a choice on what to spend your time on.

Because stocks have a tendency to go up even when they should be going down. And when you decide that it probably isn't going down, it will go down. Timing the market isn't a reliable way of wealth generation. Long term investing is.

Indeed. Since most stock are mostly held by institutional investors, prices are heavily guided by sentiment.

Except when they aren’t, see GameStop and Beyond Meat.

Prices are heavily guided by sentiment. Nobody said sentiment HAS to be tied to the entity's fundamentals. GameStop stock moved due to sentiment external to the entity itself.