You still have to deal with a lot of very different sovereign states, languages and mentalities. A Swedish customer is not going to feel much different about dealing with Greek business than dealing with a Russian one.
Imagine you want to start a business selling your own brand of briefcases. In the US, you can set yourself up to supply to the whole US (almost) as easily as you can to deal with your local state. Try organizing shipping, sales taxes, customer service etc. for all Europe and you'll probably decide to start with one or two countries.
I also feel more confident about my ability to influence consumer protection entities in case something goes wrong, 'though I'm not sure if that's actually true.
In general though I agree, (from the UK) I'm happy to buy stuff from EU places; I got my Das Keyboard and a Samsung phone (Galaxy Note, useless, sold on ebay) from Germany because it was cheaper (actually, it's also seemingly impossible to buy the Das anywhere in the UK.)
I also happily buy from the US and Canada though, as they make the funniest T-shirts, and the price ends up similar to custom printed in the UK even after import tax (not the £1.50 tax on a $20 t-shirt, but the £8 royal mail stick on any parcel with tax to pay), because of the exchange rate.
Partly, selling into another country is a bit of a headache. That's one reason why US businesses can do so well - they only need to succeed in a single jurisdiction and they'll make a lot of money. Yes, there's different state laws, but pretty much all countries have annoying state laws, and not all countries are as lucrative a market as California alone.