@IasonGabriel It's also in line with their goal of deconcentration of power. To solve problems for humanity, it's pretty hard to do so without concentrating power around yourself first.
It also allows for better specialization. They almost certainly know more about america's (many) problems than they do about... say... the UK's problem or Cambodian problems or Indonesian problems. It's humble and likely more impactful for them to build an org that focuses on a subset.