I've got a degree in accounts and economics
It's all about when it no longer can be sustained for higher gain - similar to how a pyramid scheme works but with a bit more oversight. 2008 crash was when the subprime mortgages could no longer be kept up, packaging and selling them off was also no longer viable so it was pull the investments quick. If you did it that often there'd be no faith in the markets whatsoever and you'd be harming your own potential overall gains- you have to choose when it's peaked and when to get out. It's a risky business in case you don't get it right so as always it's risk vs return.
There is also regulation preventing stuff like major shareholders just selling huge swathes to reduce the share price then buy it back cheaper, as well as stuff like insider trading. I personally don't like the idea of short selling either, even though there are protections for it.