Ann: FY21 Full Year Results - Briefing Book, page-40

The banks are legally obligated to maintain their own reserves of pristine capital assets. They already have too much cash and not enough treasuries, which is why they are swapping their cash for the Fed's treasuries already via reverse repo.
The banks have already sold too many treasuries, which is why they need to borrow them back overnight from the Fed. They need treasuries more than cash, which is also why the govt bond auctions have been so successful recently, and why yields have been dropping to record lows.