The relevant homogoneity isn't ethnicity, it's culture. Every American knows to look for white signs on the road for speed limits, to raise your hand if you have something to say in a group, and that Football is played on a field 100 yards long. These shared folkways ease the function of social life and enable commerce. You could pluck someone off the street of Montanna, drop them in New York, Louisiana, or even Guam and they could easily navigate, make purchases, share not only a language but likely interests and pastimes with locals, and order their favorite meal from a national chain restaurant.
The relevant homogoneity isn't ethnicity, it's culture. Every American knows to look for white signs on the road for speed limits, to raise your hand if you have something to say in a group, and that Football is played on a field 100 yards long. These shared folkways ease the function of social life and enable commerce. You could pluck someone off the street of Montanna, drop them in New York, Louisiana, or even Guam and they could easily navigate, make purchases, share not only a language but likely interests and pastimes with locals, and order their favorite meal from a national chain restaurant.