Productivity, culture, education, immigration.

Productivity: The USA has the most productive workforce in the world. The USA has led the world in this aspect since the 1960s. Other countries are catching up though!

Culture: Contrary to stereotype Americans work hard. As a nation America is full of highly motivated people who love money. I have plenty of family and friends in the USA and many of them are not shy of holding two or three jobs and working over twice the hours of an average European.

Also many Americans are competitive people by nature, not just in their jobs, but during their education, on the sport field, with their neighbours, etc...

Education: Of the top 10 Universities in the world, 8 are in the USA. Go to the MIT during finals and the library will be open 24 hours a day. The top American Universities also rule the world in terms of research and development.

Immigration: This is changing, but historically the USA has used immigration for it's own benefit. American research institutions and companies are not shy of pinching the world's best. And it also works on the other end of the scale, if Americans don't want to work the fields anymore a blind eye is sometimes turned to an illegal immigrant workforce who are not shy of carrying out these jobs.

Finally, you should visit the USA. I love it!!!