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Americans dominated the internet from its infancy with the majority of .com, .net, and .org domains being owned by them. As the presence of other countries grew, these tld's took on an international flavor. For Americans though, we grew accustomed to using these for many, many years and changing everyone's mindset to move over to a country specific one is very hard to do.
I suspect many .us domains (outside government ones) are parked, redirected to more common tlds, or used for personal sites. I haven't seen any search engine split out the commonly used tlds into USA only. With so many non-Americans hosting their sites here, it wouldn't give accurate results anyway.