It is much more complicated than that.

(1) Europe has a fragmented market; linguistically, product wise (2) Silicon Valley is in the United States (3) The United States has a very large amount of capital to throw at companies (4) even if you managed to succeed in the EU your shares will most likely be bought out from under you if you raised capital (5) all of the above re-inforce each other over time

Once you have that kind of an advantage it is very hard to lose it.