Disclaimer: I am not American. I have spent a total of 3 years in American corporate colonies in my country, and probably 3 times as much operating at various roles in local startups, many of which had major American funding.

I observe that even for the most capable Americans, operating a startup is at best a phase. Across the general public, it is not considered prestigious vocation. Of course, academic stars can do so, but very often that's as a side gig (unlike Big Tech, which is a valid extension of one's academic career). And for people who could both work in tech and e.g. finance, working in tech is the odd choice.

Since founding is premised on operating and having operating experience, a direct corollary is that extraordinary circumstances are needed for an American to be a participating founder (as opposed to e.g., throwing in the funding).

P.S.: Even if they are "special", the sheer size of the US leads to an abundance of indigenous founders. It's just that in my society, being a founder is a universally aspired role, whereas this appears not to be the case for US-born Americans.