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.
This is legit because I don't know if I read that here, on the internet or with my friends but it is true that they can't provide all the money you have in the bank if you wanted it all out. Why? Because banks lend your money to other people for I think loans and other things. We are talking about millions here not just few thousands of dollars because they can provide that over the counter. I you have millions of dollars on your bank account and you want it out banks can't provide you that amount.
Not just loans but there are other things that bank has which are assets that they can't liquidate. There's a high chance that the banks has money in them but I think that giving you million of dollars straight will affect them in some sort. If you have millions on bank, I'm sure that bankers don't want to lose clients like you that's why they delay it as much as they can. I don't have a experience of cashing out super big amount on a bank to the point that I need to call them before going to them but I believe that problems like this are occurring.