Language, payroll-rules and tax-rules alone can be enough to prevent a business from expanding into a market. Even when there is no foul intent, just different set of rules.

I am from Brazil and it is not unusual for companies to operate only within certain states because of this. Even though Brazil is a single country with a single language and mostly federal rules about things, there is still enough variance of laws from one state to another to cause problems.

Then you hear about Canada which has tariffs between its own provinces, but not to the US. Meaning some products are cheaper across the US border than across a province border. Figuring out how much to pay in taxes alone can be a blocker...

This is not an EU-only problem, any large single market will have friction in its internal divisions. It tends to be worse in the EU because the countries have a lot more sovereignty though (enforcement agencies tend to be local, not federal for example).

EU is for all practical means not a single market, it's a hodge-podge of rules much more integrated in theory than in practice. Which is exactly the thing causing problems here, and ironically this is exactly what the anti-EU folks complain about. Think about this disconnect: complaining about friction and requesting less integration.

> Then you hear about Canada which has tariffs between its own provinces, but not to the US.

Canada doesn't have, and should not have, the level of ambitions the EU should have. We're talking about EU 450 million people (500+ million if the UK rejoins at some point) vs Canada 35 million.

The EU should go toe to toe with the US and China. The EU is utterly dominated by US software companies and by Chinese green tech companies (batteries, solar panels, EVs).