The short answer is that it doesn’t. I think some of the most compelling cases I’ve seen have come out of South America, east Asia, and Europe.
The long answer (is really just speculation on my part) the US dumps so much money into defense that the citizens don’t have healthcare, education, or decent social services. The USA is the richest nation on earth and many billions of dollars spent on defense are classified or unaccounted for. It seems likely to me that the US is one of the only nations with a program in place to study any recovered craft so any of our allies would likely send materials/craft to the US to be studied.