It all comes down to U.S. tech companies being much more successful, mainly because of the scale of the U.S. market. Any U.S. startup can immediately address a market of $21T (GDP) without any regulatory overhead or language considerations. This causes engineers to be insanely productive in terms of economic productivity. In comparison, Germany, the largest country in the EU, has a GDP of only $3.8T. It's much harder…
BUT, I don't think it is because of the scale of the US market. As others have pointed out, Spotify from Sweden and many companies from Israel are also very successful.
I believe the real underlying reason is the strong VC ecosystem, which was established decades earlier in the US has led to the success of the tech companies there. In mainland Europe, VC is just getting started. My bet is, that we will see similar companies in 10-20 years from now.