We could also add that the true state of corporate balance sheets is more uncertain in general, as highlighted by the recent revelations of accounting irregularities in some high-profile US firms, and that American share valuations still seem high by historical standards, supported by exceptionally strong forecasts of earnings growth.
As an example of the former, when the economy is in a bad state, assuming that a particular bank is not insolvent, the central bank can lend against illiquid assets that would otherwise be fire-sold, thereby damping or eliminating the run dynamics and helping reduce the incidence of bank failure.