Shares do actually have a fundamental value, based on their lifetime dividends, buybacks and liquidation. So unlike, say, Bitcoin or the Dollar, there is something to be gained from knowing the zeroth-order reality.
When Buffet was starting out in 1950-1960s, shareholders had stronger rights and many shares did pay dividends. The worst case scenario would be gaining a controlling share and forcing some sort of payback.
Today there are many classes of shares which are disturbingly close to buying random cryptocoin.
Think about buying non-voting shares of Google,Facebook,Zynga, etc. Or how about buying whatever that travesty that ADR of Alibaba is ? What are you getting when you buy Alibaba in US?
What are you getting really when you buy those kind of shares?
You have very little of hope of getting dividends.
You have no hope of getting controlling shareholders to give up their controlling "some animals are more equal" shares.
So there is very little intrinsic value in those.
Thus the only value is the "castles in the air" valuation.
However, for those with fear of that situation, there are saner equities on the market.
You don't need lots of potential buyers for liquidity, only one for what you're trying to sell. It's why very low volume secondary markets can function and provide liquidity.
A lot of people will also obviously invest into things even when they do not consider them safe investments. If the potential return is thought to be high enough, large numbers of people will routinely invest into unsafe investments with full awareness that it's at least somewhat dangerous (whether stocks, crypto, real-estate, schemes, etc).