There are international postal agreements in place, decades old and based on the economic might of a country back then. Thus its has been invariably cheaper to mail something from China to U.S. then from a developed nation to U.S., and probably also cheaper than from US to China ( I have never done the latter)
HK Post: Fax me all of that in triplicate...
https://x.com/kjoules/status/1887398932659249540?s=19