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.
It's true that banks don't keep 100% of our money, they only keep 10-20% of our money, the rest they turn over or owe to other people. That's why if we want to withdraw all our money from the bank, it takes a week or more because the bank provides the money. First of all, that's the level of bullshit from banks. If one country withdraws all the money that is in the bank, the bank will be overwhelmed and go bankrupt. Many big banks have gone bankrupt. So be smart or look for assets to save your money so you can maintain your wealth.