One of the questions I have is why is that a requirement? Why does one network need to support the entire global populations? I mean if it's to be used as fiat

One of the questions I have is why is that a requirement? Why does one network need to support the entire global populations? I mean if it's to be used as fiat currency then I only need it to pay for things roughly in my extended community. Why aren't there seperate networks developing in distinct geographic areas?