Despite an increase in the total number of people who own shares, through such programs as the 401(k), the number of publicly-traded assets has actually been decreasing, both in the US and globally:
“The total number of publicly traded firms in the United States has fallen by half since the late 1990s.”
https://www.sciencedirect.com/science/article/abs/pii/S1386418121000458
So we have both fewer firms listed and fewer stocks in circulation, as when Elon Musk purchased twitter and removed it from public trading.
This is likely the product of repeated bailouts by the US state, starting with Alan Greenspan’s decision in 1987 as Chairman of the Federal Reserve to avert stock market declines through a combination of lowering interest rates and direct loans to struggling firms.
(Greenspan was an acolyte of Ayn Rand and was *ostensibly* opposed to state intervention in markets, but everyone should know by now that this is utter bullshit. Right-libertarians only oppose state interventions that help the poor, because right-libertarians are partisans of capital.)
Asset owners have been taught by the US state that the state will effectively guarantee that those assets will never lose value. Why sell something that will never lose value when you can simply borrow against those assets at favorable rates and re-invest those loans in owning more assets?