The Fed artificially lowered the interbank rate to zero percent for ~7 years after 2008, bought other treasuries across the yield curve, and bought trillions in mortgage backed securities. This created an environment that disincentivized saving and encouraged borrowing for both individuals and businesses alike. The market growth over the past 10 years was completely fueled by this artificially cheap credit. The economy was already extremely fragile before the coronavirus hit due to how over leveraged everyone was.