Corridas bancarias, por que el banco no tiene dinero?

Everyone pulling out their money would be a bank run (look up great depression bank runs). The bank doesn't have that much cash; they keep some on hand for people making withdraws normally, but if even a sizable minority of people all try to pull their money out at once, there'll be a major crisis.
If banks kept all the people's cash in vaults, it'd be dead cash actively losing money to inflation. Instead, they keep some on hand for withdraws, and use the rest to make loans, investments, etc so that the money isn't all losing value.