Depending on the degree to which they are leveraged, they are counting on loan payments to be able to meet the withdrawal demands of their deposit accounts.
A bank is like a multiplayer game company. Once the company shuts down, the servers are gone as well. If you can't access your bank account then you lost access to the money until it will be restored by the government. This is why we have bank runs at all. People cash out so they have a buffer until their claim is processed, not because they won't get their money bank.