edit: I am getting downvoted to hell for some reason when I'm simply trying to start a discussion based on a fact... hmmm... I don't get HN...
"The only other countries with top companies were Canada (2), the UK (2), Colombia, India and the Netherlands."
"94 of the top 101 companies are based in the US."
...they have to for at least 3 months to be a YC company, and after that the question is what is the compelling reason to move?
Why not both?
There's a probably at least a bias to English fluency, if not a blanket rejection of non-English applications.
There's been a long-standing bias towards teams, which mandates a level of charisma and social skills that many talented engineers struggle with. For a long time, YC completely excluded solo founders.
And I believe being on-site is another requirement, which is classist. People from other states and countries who don't already have affluence probably don't even bother.
YC requires a 3 month relocation to Silicon Valley.
That itself creates a pretty strong bias.
There are probably language, cultural, and other biases at play, too.