It's definitely possible and easy for a company in EU country X to provide a service to individuals or companies in another EU country. What makes you think otherwise?
EU investors will invest in a working idea.
We have far fewer ridiculous startup ideas or companies that can't scale, but the flip side is that we have fewer winners too.
Investors in the US will throw millions or even billions of dollars at an innovative new business, even if it doesn't make that money back for years. They'll invest in consumer facing businesses, like consumer tech products, apps, social media sites etc.
EU ones won't do that. They'll invest fairly tiny sums in safe, conservative businesses with a proven (but arguably limited) business model, and that won't be enough to get the kind of unicorns Silicon Valley specialises in.
In the EU the culture is much more cynical, and the belief in individual impact is much lower. There's also more of a sense of there being things that one just does not try to exploit, types of business that could be considered morally distasteful (see the healthcare industry for the clearest examples of what I mean).
These are very broad generalisations, they don't apply evenly across the US or the EU, and while they all sound quite negative they are really trade-offs and it's up to individuals to decide where they prefer to sit on these trade-offs.
https://www.mightyrecruiter.com/recruiter-guide/at-will-empl...
Almost any american state is a vastly better place to start a company than almost any european country.
Talent availability and local startup ecosystem don't count for anything? All that matters is that you can dismiss employees for any reason at any time?
The average non-contractor salary is much lower and it's rare to find startups hiring contractors, I assume since they don't have as much money.
For all that America likes to proclaim how varied it's culture is from state to state, I'm a 2 hour train ride from switching currency and language, in addition to substantial cultural norms.
Such things seem much more significant to me than notice periods.
The USA is a much more attractive environment for investment and hence startups and the EU would struggle to offer the same without losing its soul.