Look at the USA as an example. The difference with the EU is that the USA are vastly more homogeneous and I believe there is very little opposition to redistribute wealth from richer to poorer states. The EU is fragmented, almost nobody wants a tight political and fiscal integration. What would be of the USA if state governors were much more powerful than the federal president, to the point that few people would know who the president is and what he can do? This is the state of the EU.
But this would massively hurt the weaker and smaller economies though. Businesses congregate where other businesses are. Workers follow. Brain drain has been a big problem in the EU. If EU countries all have to adopt similar taxation schemes then why would you start a business in Romania if you could just start it in Germany instead?