Reinforced by the apparent reality that mostly what people outside the USA want to talk about is the USA. Even on HN, where we have a large contingent of Europeans participating, we don't hear too much about Europe. It's almost always about the US.
I'm from Europe and sure, one of the main reasons why I come to this US-based site with a majority of US readership is to find American viewpoints on issues, and engage in discussions about the USA.
This doesn't mean that I spend my life wanting to talk about the USA, though. I talk much more about my own country but it's on sites with readership from that country (and in real life, of course) so you're not aware of it.
And many of my compatriots don't talk about the USA at all, you just don't see them because they don't hang out in sites like this at all.
(Maybe this is already what you meant, that when an American interacts with someone from outside, they often want to talk about the USA -which is not the same as saying that people from outside often talk about the USA in general-. But it's not clear from your wording).
Of course this is a US-focused site to some extent, but the US just has a big contiguous language community. It's comparable to all of Europe, not individual countries.