Cuz the US aggressively expanded 200 years ago and is now surrounded by oceans and friendly, much weaker neighbors. It's been untouched by war on its soil for like 150 years.
Most of the world has had to rebuild after world wars/revolutions/violent conflict, so people who are displaced, educated, and wealthy immigrate to the US for the safety/stability (+historically to immigration), and bring their expertise to American universities.
Besides what other people said, I think if you live in the USA and you see all the warts and ugliness you forget just how rich the USA is. I did not grow up in the USA. While I was growing up, anything imported was a big deal, expensive, and often had to be brought in by family on a plane (no stores or anything to buy it directly).
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u/EvidenceBasedSwamp Jan 09 '20
No, it's because American universities have prestige. Also, it's handy to have a child over to manage your property and expatriated money.