Because America has the resources to educateWhy do one third of the scientists on Earth come from America?
And because our goofy college subsidy system enables people get PhDs to avoid the real world
Not to mention these scientists are a very small part of the population... most Americans are not good at science, not interested in science, and often times very anti-science. The US leads in science but it also leads in religious fervor, in the context of the Western World