Here’s how it works. Governments require banks to hold capital against the loans that they make, anticipating that in the normal course of business, some of the loans will not be repaid. The riskier the loan, the more capital that needs to be held in reserve.
However, under international rules negotiated by government representatives through the Bank for International Settlements (BIS), government loans fit into a special category that has a 0% risk requirement. That means European banks do not have to hold any reserves against loans they make to European governments. That’s right, politicians implicitly promised banks that governments would never default. And, given the opportunity to make “risk free” loans that require no capital commitment, bankers purchased mountains of government debt.