Maybe people aren't having their ideas ignored in Europe. Maybe they're afraid to leave because there are inadequate social safety nets (wait). Maybe European ideas suck? Maybe it's too hard to start a business so lots of people don't; or they have an easier time leaving the country and starting a business in the US (which we'll call American success, even if it's founded and built by immigrants).
Anyway, if that path was more feasible, businesses would need to pay more to encourage people to stay instead of going alone.
I know “immigrant thinks america is great and welcoming” doesn’t fit the “make you angry so you click and share” narrative currently present online with regards to immigration, but it certainly fits the lived experience of myself and many immigrants I know, both legal and illegal.
[edit: forgot to write, I agree with you about the difficulty of starting a business in Europe vs the US, as well as maybe the cultural pressure in favor of running businesses in the US vs general attitude of discouragement in the UK. ]
The UK market is not big enough to bear big successful companies (by HN and VC standards). The first thing a UK company has to do is extend to the US to be able to grow.
If you were into video games in the 80s and 90s, there are lots of stories of developers starting their own thing; the barriers were quite low (a lot of people distributed floppies in ziplock bags going to stores talking to store managers); and you see that in UK and US, and sure there's more in the US because there's more people. But I can't think of any internet sites/products that started that way in the UK.