80% of companies in the S&P500 over time produce zero return. 20% of companies account for ALL of the returns. In the past these would've been driven by massive growth stocks like Amazon going 1000x over a couple of decades. Now, the main reason to IPO a tech stock seems to be because the company has reached its growth potential and you want to flick it off to derisk early investors, and often it plummets as reality sets in. Meanwhile, high growth stocks are captured in pre-IPO markets and the gains are all accrued to private investors.
I think the S&P500 is an entirely different beast to what it was 20 or even 10 years ago as a result of this.
If you were a family office your goal would be to get exposure to pre-IPO growth stage stuff as well as the public market.