Try searching for any life knowledge on the English web without specifying the country. E.g. home repairs: I'm pretty sure Brits don't live in those funny cardboard houses.

The underlying distribution doesn't matter when there's more activity from one stratum. The same way how it doesn't matter that the ‘average age’ of a Reddit user is in the twenties, when each teen on there posts and comments every ten seconds.

I don't know where Brits, Australians and English-speakers from all other countries are hiding, but they probably just use local communities away from the annoying USians. English web that you find by default is US-centric. English Wikipedia even has a special maintenance plaque for articles that are US-specific and should probably be rewritten someday—could as well just slap a sign like that on the whole English web.

When I see complaints on this very site about Californians assuming by default that everything is in California, I laugh heartily, tingling all over with schadenfreude.

This is self-reported. Consider that 90% of Croatians reported they speak English, but this is mostly the level required to guide a tourist to the closest bar around ;)

EDIT: my bad, you were talking about native speakers.

EDIT2: well, no, my point still stands: the Total includes self-reported and vastly inaccurate additional language speakers. But based on how US is 2/3 of all native speakers, and that there's definitely many fluent-English secondary-language speakers out there, US is definitely not dominating English-language web.