Your bank doesn't actually have most of the money in your account

Under fractional-reserve banking, a bank keeps only a fraction of its deposits on hand as reserves and lends the rest out, which is what actually expands a country's money supply. It works smoothly as long as only a normal fraction of depositors want their cash back at once; if a large share demand it simultaneously, a solvent bank can still run out of ready cash, a self-fulfilling panic known as a bank run, which central banks now exist partly to prevent.

— Wikipedia contributors, Fractional-reserve banking — Modern banking systems