Silicon valley has the benefit of long term investment and also being a single centre for all of the US market. EU single market is not nearly as well integrated as the US in practical terms e.g. basic language barriers.

Decades of "infrastructure and community" is not easily copied else where and is a real advantage for the US. Europe had lot's of "Facebooks" but non grew out of the local market to be able to gain a global advantage.

Silicon valley is not easily reproduced elsewhere in the US either.