[forbiddenvoid]

Saw the title and was immediately confused about why the answer to this question isn't just money. Turns out it is. US universities are benefited by subsidies from the US government and large grants from the private sector. Much of that capital is a direct result of the advantage gained by both US colonial interests abroad and by the US not suffering the infrastructure damage most of the rest of the wealthy world experienced as a result of WWII.

Edited to add: the brain drain from Europe to the US during WWII is also a factor. We ended up with a disproportionate number of foreign scientists during that period as well.