The distinguishing factors here aren’t simply European regulations and taxes.
I know of American VCs who have been just as ambitious investing with U.K. startups as with American ones.
If we are to take the content of your post at face value, that thesis is wrong. It was due to sand hill road being where sand hill road is. That may have something to do with taxes and regulations to a degree, but that isn’t a sufficient explanation.
and within the US, "why SV?" might simply have another orthogonal explanation.
If that were the case, you would have to see unicorns in Delaware and Texas and many other places in the US, especially since many of these have a much more lax tax regime than California or New York.
The fact that unicorns are almost exclusively bred in two cities in the US actually suggests that the influence of the tax and employment regimes is overwhelmingly dwarfed by other factors.
No offense to Europeans, but I've only heard of Spotify.
[1] https://www.statista.com/statistics/813906/spotify-number-em...
The reality is that it's just not possible to start these kinds of Unicorn companies anywhere except in the USA. The EU's extreme policies contribute to an overwhelming burden of tax and employment regulations, which results in risk-averseness. So entrepreneurial Europeans go to USA to start their company.