There is a saying that goes "share your wealth with us or we will share our poverty with you". The whole point of the EU is that the stronger members bring up the poorer members so that they don't dissolve into financial chaos which tends to have other inconvenient outputs.
That's actually the problem with the EU. Poverty doesn't go away just because you reduce trade barriers. If it did, NAFTA would've turned Mexico into a shining beacon of democracy. You need political and legal reform to disperse the conditions that are causing the poverty.
The EU does take some steps towards this - e.g. harmonizing product standards. But for the most part the EU countries are insisting on political independence. That's like trying to hitch up a bunch of horses of different athletic ability to a single wagon under the premise that the faster horses will bring the slower horses up to speed. What really ends up happening is the slower horses end up getting dragged along, and the faster horses end up having to work harder (e.g. Germany and Greece). You need to condition the horses until they're of similar fitness (i.e. political reform until they're of similar economic strength) before you think about hitching them all to the same wagon.
The U.S. tried what is basically the EU approach in the 1700s when it first won independence from Britain. Mostly because of the bad aftertaste of the overreaching British Monarchy, each state wanted to govern itself as if they were separate countries. That lasted about a decade before it became obvious it wasn't working, and a stronger central government was needed if there was to be a union.