Because the dollar is the world reserve currency, and lots and lots of large pension funds, university endowments and family offices spend tiny proportions of their assets on VC (as it's expected to be uncorrelated with market returns).

VC's tend to be look a person in the eye and judge his (almost always) character. They don't like travelling, so most of their investments are made in the Bay Area.

Coupled to this, there's much more availability of angel funding because of the previous unicorns (paypal, eBay, Google, Facebook et al) so it's easier to get started.

There are definitely some cultural issues at play also, but fundamentally the reason that the US has such a high proportion of tech giants (which seems to be what you're focusing on) is the low cost of capital caused by the dollar's exorbitant privilege.