EU member states want all the benefits of integration without giving anything up, especially things that they consider aspects of national sovereignty.
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I'll give you an example.
I'm from Romania. In Romania in 1990 there were no private enterprises, so everything was state owned. There was a HUGE demand for commerce/retail, so small local businesses popped up. They were very basic because everything was brand new and expertise was extremely scarce. No one knew anything about modern supermarkets, supermarket chains, banks, loans, credit lines, basically everything related to modern market economies.
As Romanian headed for EU integration, foreign invested started... investing. In retail that meant that because Romania is fairly populous by EU standards, everyone was tempted by a potentially big and growing pie that was almost void of serious competition.
So we ended up with Lidl (German), Kaufland (German), Metro (German), Carrefour (French), Mega Image (Romanian, bought by Delhaize: Belgian), DM (German), Cora (French), Auchan (French), Profi (Romanian, bought by Delhaize: Belgian), Selgros (Swiss), Penny (bought by REWE: German), etc.
I moved abroad somewhere close to both France and Germany. At a company I worked for, I had both French and German coworkers.
At some point the Germans were talking about Kaufland and I started chatting with them about it. Some French people mentioned Auchan and I replied to that too. After a few more replies about German and French supermarket chains, one of them said: how do you know so much about both French and German supermarket chains?
And I told them:
We're right next to Germany and German supermarkets stop at the border. There is no Aldi in France. Most French people in the neighboring French towns haven't even been in an Aldi.
We're also next to France and French supermarkets stop at the border. There is no Carrefour in Germany. Most German people in the neighboring German towns haven't even been in a Carrefour.
In Romania your big companies bought everything local (or displaced it), there are hardly any locally owned stores so I guess the upside is that I know all your supermarket chains.
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Repeat this for banking, IT, whatever. Europe is super economically segregated even with the EU operating. Each country has all sort of hidden protections for its local businesses. I can't really tell you what they are exactly, but in practice you can feel them. Each country has its own version of X, Y, Z businesses. Small versions of everything, instead of few really big and competitive businesses at a global level.
This even impacts US companies. You can't get a European Netflix subscription. You fly from Romania to France and when you land and connect your tablet to the internet, your Romanian Netflix account switches over to France and they delete Mad Men from your downloaded episodes because Netflix France hasn't licensed Mad Men (old example).