These days, that's mainly because we import so many East Asians, Indians, and Europeans into our postgrad programs.
We import them from American universities though. Our high schools are way more broken than our colleges.
That disconnect has always confused me. How does country manage to maintain the best universities and among the worst high schools in the first world?
"Imports" have always been our research strength :-). Thank you Germany for going all crazy in the 30s giving us an era's worth of top-tier scientists.
Except those scientists stayed in America.
Today's crop of Chinese and Indian scientists want to go back to China and India.
The claim is that the undergraduate education component of the US university system is broken, not that the basic research component of the US university system is broken.