Because, by definition, the most extraordinary achievements are only attainable by the small subset of people with the rare, innate ability to accomplish them — that's what makes those achievements literally extra-ordinary.

If anyone could have the athleticism of Michael Jordan or the intelligence of von Neumann or the musical ability of Mozart or the artistic ability of Michelangelo through hard work alone, there would be a lot more people with those respective skills, and the skills would no longer be considered extraordinary.