They do go bankrupt from time to time, for that exact reason. Most of the time, enough of the sleeping normies "keep their money in the bank" so that doesn't happen.
Discussion
So if any bank in the world could go bankrupt at any time, why are there banks signing up to fractionally reserve and lend?
Why don't they see this danger and avoid it by keeping enough reserves on hand to meet all possible withdrawals?
Why would anyone want to engage in fractional reserve banking? Because the profits can be immense. Why don't they keep more reserves on hand? Because that reduces the profit potential. And some banks are indeed more restrained that others -- are less, shall we say, extended, than others and do have larger reserves. Eventually, there were (pretty modest, like 10 per cent.) reserve requirements imposed on banks in hopes of preventing the recurring "panics" that had ruined many an innocent man, but these requirements have varied and banks still go bust. And parts, at least, of this parasitic overclass buy legislators and get "bailed out" when needed. Nice work if you can get it -- provided you have a brain too small to understand that you are engaged in predatory fraud, or a conscience atrophied enough that you don't care.