So if banks are super desperate for Fed funds, but actually have Treasuries to put up as collateral to borrow those funds in the overnight market, why don't the

So if banks are super desperate for Fed funds, but actually have Treasuries to put up as collateral to borrow those funds in the overnight market, why don't they just sell their Treasuries instead, or let them mature, and hold more cash? Hasn't the Fed for years now been literally paying the banks to do just that with IOER?