It's because it's all electronic now, you don't really need that much money to prove that you've got money, just a piece of paper and a computer that stores your information and you're good to go, that's how banks are working ever since and it's called fractional banking, if I recall, the money that they receive from customers are quickly circulated or something like that and that they have only a limited amount of physical cash daily, I might be wrong or that's only how small banks or bank branches operate.
they only make some percent of cash available for withdrawal, when people want to withdraw big money in cash they should inform the bank before hand and make the bank trying to make it available.
the rest of the money when people are saving in the bank are being used to grow the money, giving loan to people and many more.
thats how it works, bank operational needs huge amount of money, so its kinda understandable because if it only runs from the charged monthly service fee im sure bank can't be as big as now and can't make many buildings for their office, its just that this sometime implicates a problem when there's money rush, when people trying to withdraw their money together massively, some bank will collapse, but honestly there's no point doing that, just trying to harm their own economy.