Just because they are overstaffed as a whole, does not mean that they have the right skill sets available to them. I would expect that many people with generally marketable skills have left and that the over staffing is in the specialist banking areas...
People with marketable skill such as software engineers etc... would have been recruited into the banks on lower than normal salary levels on the promise of a good bonus instead, since those days are long gone, these people will have moved on too.