The value of US stock market is supported by money from all over the world.
It is completely bullshit to compare the value of stock market to US GDP.
It makes more sense to compare stock market value to the world GDP.
It is much easier these days to trade in US stock market than 60s and 70s.
People in Europe, Canada, Japan, Taiwan, Singapore, Middle east, Brazil are trading in the US stock market.
No matter retail traders or big institutions, money is pouring into the US stock market