The way you look at it is wrong. The bank is not an overflowing water source. Imagine it more like a small pond that someone owns. It dried up sometimes and gets filled if there's ever rain. Now people living around the pond use that water for their farming and other stuff. They pay something more valuable in return when they are harvesting. If there's no rain, people will use up all the water making it dry leaving that pond with no water. But the owner will get a valuable return in the end no matter what.

This is what also happens with the banking system. They are not lending out their own money. They are lending out money that people deposit in that bank. And they lend out all of them immediately. Because if they keep it in their possession, they won't be able to make profits. In the end, when everyone at once asks for a withdrawal or makes a huge withdrawal, they are unable to give it back to them. That's how it works.