Which is exactly what Citi and BoA are doing now. It's an easy way to get really profitable really fast, using leverage, and pay off TARP loans (which is in everybody's interest) if / when financial system is stabilized and bear raids or run on the bank are no longer in the picture. That would unclog the credit facilities of the banks, which have been already thawing due to lower spreads and LIBOR rates.
The spread between corporates and treasuries simply was and is the crisis. If the Fed wanted to address it directly they'd just arb that spread directly. Instead they continue to do it indirectly, wanting private actors to take the lead. That is delaying things, but sooner or later it turns.
The original losses have all been written off long since. All the big banks have been cash flow positive throughout - the "losses" are all marks and additions to loss reserves, not cash going out the door.