Banks don't store the cash, they lend most of the cash to other people. If the other people don't pay back or the other people don't exist because the bank is a scam, there is no cash to give to the people that put money in the bank.
You can create a 100% reserve bank that doesn't lend money, it can be created in real life or in the blockchain. (There are a looooooooooooo...ooooooooooooot of regulations.) But this bank will not pay interest, and must have fees to get money to pay the employees, buildings, ... So people will not put the money in this bank.
Most internet exchanges work like this with a 100% reserve, but we all know the story of MtGox.
[Hi from Argentina! We have a global bank run every 10 years.]