... according to QS Global Surveys. A disreputable organization who won't open their data and whose rankings are based at least 50% on reputation surveys (!!), of which only 2% or so are responded to. Is it worth mentioning that it's based in England?
I'm not saying the US should have more institutions. I'm saying that it's highly unlikely that four of the top 10 institutions worldwide are British.
Regardless of what you think of QS. I also think it's silly to have uni rankings, but the way to think about them is that they simply construct a ranking that shows what people expect based on reputation. If you don't have Harvard and Cambridge in your top tier, your ranking is wrong.
But reputation is not a just measure of quality. It's mostly a measure of whether someone has heard of the institution at all, or knows anything about them. Similarly, US News has reputation survey based measures for its terrible rankings, and is roundly criticized for them, as they largely shut out better but newer but less well known institutions.
I suspect the same thing goes for the corruption perceptions index. People conveniently drop the perceptions part, and they get their corruption reputation from last year's headline.
Northern Italy went through an awful lot of tumultuous history between 1450 and now that England didn't experience: it was cut up into lots of little fiefdoms, dominated by several external powers, and generally kept under constantly changing iron thumbs.