>This would have been "safer", but lost them money.

In a short-term sense, but the point of short-dated maturities is not to produce yield but rather provide liquidity, which allows you to purchase higher-yielding 10+ year notes in the event the interest rate rises.

They would've actually gained money by (1) not being forced to sell assets at a loss, thereby leading to a run and also becoming insolvent and (2) e.g. used the maturing short-dated bonds to purchase 3-year treasuries at today's 4.1% rate, rather than their shitty 1.8% 10-year notes.