Originally, there was a clearly defined allocation of responsibilities, with the Maastricht Treaty stating that fiscal policy was a matter of national ownership. And the no-bail-out clause (which is today Article 125 of the Treaty on the Functioning of the European Union) went as far as to explicitly prevent member states from assuming liability for each other’s debts.
Action and liability, then, were located at the national level. As a measure of last resort, the no-bail-out clause states that it must be possible to impose a haircut. That’s something that creditors need to be aware of. I imagine the architects of monetary union will have thought this an unlikely scenario.