Earlier quoted context omitted.
Because the Fed isn't there all the time. Under normal circumstances, you can't just sit there. Aside from competition, you won't retain quality employees.
But they are now, and that's what matters. When prices fell the first time, it didn't even take a week before the government passed the largest stimulus ever, 80% of which went to corporations. Investors know that the government will do anything to underwrite their risk.
The FED only steps in for once in a lifetime risks that impacts the market broadly. Any risk specific to individual firms, which is all the risk between these broad market events.