The social union happened anyway via the backdoor (as always), when the ECB decided it would do "whatever it takes" to stop bond rates rising on Eurozone countries. That opened the door to arbitrarily large wealth transfers from richer to poorer countries, which were then ploughed directly into the benefits system (Greece is an example of this).

So, where are the common social (minimum) standards?

Regarding Greece, mind that Greece debt was totally covered by German restitution obligations, which is also why banks (among them prominently German banks) rushed to propose credits to Greece, since this was backed by the German economy, supposedly a safe bet. It was only when Germany allowed itself a unilateral haircut that this became problematic. (Also compare the London Agreement on German External Debts, where the payment part has been factually canceled in favour of Germany. As an effect of this, while, e.g., the UK was repaying lend-lease debts right into the 21st century, significantly hampering its economy, Germany came out this without any war-related obligations, even – compare the Greece pension treasuries – taking ultimate possession over some of the war-time "acquisitions", while enjoying recronstruction aids at quite a scale. There's a major imbalance in this.) Moreover, the common currency provides more developed industries access to poorer markets on equal footing, thus providing the means for a transfer from the poorer, mostly Southern economies to the richer, Northern ones. Any attempt at a common control of bond rates just mitigates the effects of the common currency for those countries. (Opposing this is much like wanting to eat the cake and still have it, which isn't prone to work out well.)