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.
The bank deposits the customer's money and lends that money to other customers, thus the bank makes a profit by managing other people's money. So if all the customers withdraw from a bank together then that bank will not be able to pay all the customers together. The bank will go bankrupt. So you can say that the bank has no money. Banks only change hands of money. And earns profit by lending business with customers money