Because your money is used to be owed to other people. so if you have a lot of money in deposits or ordinary savings. and want to take it all, the bank will have difficulty if you immediately give all your money.
So the bank will give time to provide the money you want to take maybe sent from its head office or the person paying the debt.
many people don't know the reason why they are saying that there's no money in the bank it is because in the bank bankers use our money to trade and also borrow some people secondly the Central Bank of that particular nation maybe they are lacking printing of a new currency or the money is being hold my politicians for their own selfish interest this is one of the things that scarcity of money in a country so you cannot blame bankers that much because maybe they probably might come from the government not to them but sometimes bankers use our money for trading exchange mostly