Simply put, when enough people deposit money into their accounts, banks will simply take the money and lend it to someone else. It is not “their” money, but crunch enough numbers and do enough predictions and you might make it out.
That’s why “run on the bank” is such a feared things - if everyone starts withdrawing cash or sending it to a different bank, the bank can’t really do that because they don’t have the money.
Of course, but that doesn’t mean one should get it. If you are gonna haul wood once a year, you are not BUYING a log trailer and just tow it behind you vehicle through the city all year - like a sensible person, you rent one for a day or two.