They've been injecting so much cash... that's a lot of fucking money. I think when they slow down, the market will crash. Money needs to be in circulation, and this is the only way they can get it to circulate (although, tbh, if it were in the pockets of the people, it'll probably circulate better).
You can also check the velocity of money in the M2 velocity chart [1], which shows the ratio of transactions to the M2 supply. The M2 supply is the M1 supply plus some other forms of money. What the M2 velocity chart shows is that people are holding on to their money in some form and not transacting with it as much as they have before (i.e. a slow down in the velocity of money). Like, it's getting close 1!!!!
So all that cash that the Fed's been injecting hasn't been able to speed up the M2 velocity back to pre-COVID times let alone pre '08 recession times.
The velocity chart doesn't look too good IMO.
[0]: M1 money supply https://fred.stlouisfed.org/series/M1SL
[1]: M2 money velocity https://fred.stlouisfed.org/series/M2V