You can't compare this at all. It is hard to even begin to explain the multitude of problems you get from dealing with multiple countries if you haven't actually lived in different parts of Europe and know what it is like speaking english as a second language. I'd say I am fluent in english, when I am in the US Americans normally don't notice I am foreign. But that doesn't mean there aren't lots of obstacles. My english is full of holes. I know lots of computer science terminology. But if say I wanted to buy stuff and home depot or the grocery store or whatever I would be lacking a lot in vocabulary because in my world I don't use english for those kinds of settings.
This will apply to any European. If any of us were to say create a web service for house sales, mortgages etc we wouldn't know what the terminology would be. And practices and terminology would vary widely from country to country anyway. My parents generation can move around Europe fine using their english as tourists, but if they had to actually use government services, buy a house, a car etc in english they would have a lot of trouble.
If I start a company in say Norway, then there are no registers in Germany listing my company. My bank accounts would not be directly accessible for transfer for a Germany business. You typically have to setup offices, bank accounts, marketing etc in each country you operate in.
Culture is a big deal. Typically here in Nordic countries we sell a lot to each other. Norwegians and Swedes speak a different language, but we usually trade a lot with each other and companies are usually quick to start utilizing each others market. But that is because culture is much closer. As a Norwegian you understand to a lot degree how Sweden operates. We get Swedish news, movies and know roughly what is going on there. If we were to sell in Italy instead e.g. it would be a completely different ball game. We know little about what is going on in Italy. We don't follow Italian news or watch Italian movies. We don't know Italian culture, language and business practices at all. Italians have completely different tastes from Scandinavians. E.g. I noticed when visiting Italy a few years that they type of clothes and the way Italians buy clothes is completely different from Scandinavians. Italians wear fashionable clothes, high heels, elegant coats etc. Norwegian girls are usually walking around in training gear, even when they are not training.
Compare this to the US. I've been to states as far apart as New York, Hawaii, North Dakota, Utah, Texas and Washington. And clothing and consumer habits are actually not that different. Perhaps an American would disagree but from what is immediate obvious to the human eye the differences are lot smaller than say between Norway and Italy.
It is the homogenous preferences of the consumers across 320 million people which creates huge market opportunities for American companies which does not exist to the same degree in Europe. Of course this is changing in Europe as well. Habits are getting more homogenous but we are still far behind the US in terms of appearing as one coherent market.