Often only minimal shares are floated on the public market - 5-10% now is not unusual. Also, founders keep priority shares to keep the company.

So IPO is not particularly a liquidity event for investors as much as a valuation/pricing event. Indeed, the tech IPO's that have done the worst were the ones where shareholders wanted liquidity.

Clearly none of the multi-trillion dollar companies could find a buyer now if they really needed to sell themselves, so they're not really "worth" that much. (Nor are their founders, who can't sell their shares without tanking the stock.)

So these stocks are more like derivatives: a way to bet on the future where betting volume is huge relative to the underlying asset.