Why are most of the top universities American?
It started in the 1930s, when Nazi Germany started to fire Jewish professors and other people who opposed the regime. Many others left voluntarily as well. Germany had the best universities before this period. The United States ended out getting more of the refugees than any other country (e.g. Einstein) and this started the ascendancy of American universities.

Then World War II happened. Germany ended out with their universities basically obliterated, through the actions of the Nazis and also from the war itself. Japan's universities were also very damaged. The US on the other hand, not only benefited from the refugees but also from the war effort. A lot of money and effort was put into the Manhattan Project and so on, and this infrastructure was used after the war. Other European countries did not really gain as much... some suffered extensive war damage and others like France were at least occupied. Britain's academic institutions were (basically) intact but they were losing ground to the Americans.

After WWII the Cold War happened, and universities were highly supported by the US government during this period, building on the earlier growth. The US had a lot of financial resources and were willing to use them to help universities. Also, American universities in recent decades have been quite welcoming to foreigners, and the increasing prominence of US universities in turn attracts immigrants. So this has become a self-perpetuating phenomenon.