The drug dealers have cash. At certain moments cash in a bank vault is of supreme significance to the existence of the bank as a viable institution. In order to prove that a bank is failing, it is necessary to show that it has experienced huge losses, either from sudden significant withdrawals, or because it made very accommodating loans, or as a result of the failure of one or several ventures in which the bank had invested heavily. It's more likely that a crisis occurs when the bank's reserves are inadequate to meet the requirements of the Federal Reserve. It is at those moments, when the bank's reserves are dwindling, that cash in the vault of a bank is a life saver.
Cash is the bottom line to everything in banking.
It should be remembered that for every seven dollars in deposits, one dollar has to be kept in legal reserves. That means either government bonds or other government-supported securities, plus cash in the vault. Every bank tries to keep the barest minimum of legal reserves. Having in excess of the legal requirement is unprofitable. The legal reserves of a bank do not produce interest for the bank.