The United States is not listed because people in the United States do not request information from the State Department to travel to the United States.”
So, other countries see a different US Department of State web page?
Other countries presumably get advice about traveling to the US from their own foreign ministry, for which they pay taxes to provide services to them.
I tested the theory by going through an anonymizer that was going through Australia, and saw the same home page. So I don’t think there is any particular localization of the page going on.