If you compare banks to river, they only distribute water, not store it. Likewise for those of you who intend to store cash (limited supply), it only converts it in digital form into a few bytes of storage, well their storage isn't limited to that digital number. Meanwhile, your cash is used for liquidity in their business.
That's why, they (and protected by the government) limit cash withdrawals per day and can change at any time to balance with their liquidity, if they fail then what will happen is a liquidity crisis like your local bank.