There is literally a "world" of difference between a collection of trade-aligned sovereign nations, and a single nation of autonomous economic regions. As you mention, it's an accepted part of daily business in the US but everybody pays taxes to the Fed at the end of the day. All governed by the same congress, president, and a coherent political dialogue throughout. That's not to say it'll always remain so.

The particular problem with the EU is that it's a system of "good faith" agreements that has until quite recently been quite weak against cynical attacks.

This kind of dealing undermines the spirit of the EU accords and you can expect it to be dealt with in the course of time - probably through tax harmonisation, which wouldn't then be a million miles away from the american model.