Earlier quoted context omitted.
The odds of a commercial bank going bust is higher then Germany going bust. Commercial banks only need to keep a few % of the money deposited at them on their books (its called fractional reserve banking), the other 95% or so can be invested or lent out elsewhere. There are regulations that restrict the risk, require hedging, insurance, etc but those don't help much if the insurer or a significant amount of the borro…
Banks are allowed to loan out the money, but they don't have to , and could take business from the negative-yielding government bonds by just leaving the money alone, in their electronic reserve account.
They could keep it in the Fed, but the Fed charges money for the privilege (and I assume so does the ECB and other equivalents)