I think it's fair to ask why there aren't any silicon valleys in Europe. But you should also ask why there aren't any other silicon valleys in the United States.

A better study would be to take silicon valley out of the picture, and then compare the "best of the rest". How does the start up scene in Copenhagen compare with Seattle? Berlin and Austin? Boston and Oxford?

I have a feeling US based cities would compare favorably, but not as dramatically, and perhaps at a higher social cost.