The root DNS servers are
NOT all in the US.
There are 13 root DNS servers in the world (A through M), and the bulk of them are located in the U.S., but there are many in Europe and Asia. If the rest of the world had to talk to the US for domain name resolution, the Internet would seriously chug for everyone outside of North America.
The 13 servers are comprised of clusters of many smaller servers that are spread out to balance the traffic load, and for security. For example, "I" has servers in London, Helsinki and Miami.
This site lists the locations of all root DNS servers:
http://www.root-servers.org/
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