The last thing we should want is the entire world consisting of one giant country. Take America for instance. If you don't like the culture, abortion access, state tax rate etc. there are any number of other states to move to.

Same with countries. China too oppressive? move to Switzerland. Tired of the nanny state of Singapore? Head out to the great American west. In my area, I don't know too many people who'd want to live in say, Alabama for instance. It's a good thing that we have differences in thought and lifestyle. Who knows, maybe one day something like what you imagine may just happen...