Probably because US companies buy European ones.

See Skype, Nokia, Shazam.

China’s doing it too. They bought ARM shortly after Brexit.

I don’t doubt that there is a migratory factor, though I don’t believe it’s primarily due to salary or other compensation as I assume you’re inferring- a cornucopia of tech giants is going to attract talent from the world over.

So a consequence of centralisation is more centralisation.