Which is the way it's supposed to work. You keep enough for daily transactions, because the expectation of multiple large scale withdrawals happening either in short succession or simultaneously is the most unlikely scenario to happen during operation. A bank keeps records of every account's value, but at any given time only has enough cash to cover one fifth of all of the money it has on record to be in those accounts. In other words, the bank's physically only got 20% of the money it has on the books. It has to work this way because there's no way a bank could hold all of the money it's customers are said to have, either because of physical space constraints or because there's literally not enough money in existence to cut it out of circulation without creating ridiculous deflation. The change away from the gold standard changed this quite a bit, and so has digital banking, but the numbers in your account are still backed by something that tangibly exists.

> the numbers in your account are still backed by something that tangibly exists

Only if you consider fiat money that can be printed in arbitrary amounts by Mr. Bernanke's famous printing press to be "something that tangibly exists".