Any financial advisor worth their salt has basically only one piece of advice for a retail investor saving for retirement/education/mortgage down payment: "Buy and hold. Buy all the way up, buy all the way down." And it works. In the end, scarcity dictates that the value of every asset will always go up in the fullness of time.

So why don't pension funds and insurance companies just buy all the time? Why would they ever sell if, in the long term, buying and holding always wins?

If the S&P tumbles one week, us retail investors are strongly advised to keep buying. But the only reason the indices like the S&P would continue to crash is because the big guys, the people with the money, are getting scared and selling. But that seems idiotic. They're not just richer than us, they employ the smartest finance brains in the world. You would think they'd know better... but yet they sell with the dip, which is the one thing that retail investors are urged to not do.