There's a lot of unknown right now. Money is pooling in the hands of the 1%, and isnt really moving through the local economy very well, specifically in big cities in blue states. If those economies start to fall apart due to continued lockdowns, even with a government bailout you cant just magic up new businesses to replace those lost, nor magic people back to the cities. You cant magic away all the debt, missed payments, and social fear.
The US stock market cant survive on just a few megacorporations. All the companies on the stock exchange regularly do business with small and medium sized businesses. If those dry up there will be a dramatic decline in revenue. Even if this is a temporary hiccup, consumer debt of all kinds is at an all time high right now. Any sort of downturn in the economy would spur on a domino effect, from CC debt to housing debt to auto loans and student debt. All these bubbles would be at risk of popping in the event of a slowdown, or rampant inflation cutting consumer buying power yet again.
A domino effect on debt bubbles would cause a stock market crash.