What's with the homogeneity in Avatar?

No.113861635 ViewReplyOriginalReport
Because of Corona-chan knocking on my door I've been rewatching the series from TLA to LoK and I suddenlly realized something.
How can FN, WT and EK during TLA be so homogeneus when it comes to the people that inhabit them? There doesn't seem to be any mingling of people from different groups. Each seems to be so racially/ethnically pure it would make Hitler envious.
One could say it's because of the 100 year war, but that doesn't really solve anything since it would only explain FN being racepuristfags. What happens with the WT and EK?
With all of this I don't mean to ask why people didn't move during the war. My point is: what about the people BEFORE the war? Those gorillion years between Wang and the Genocide. There was no movement of people? Commerce? Any of the classical reasons for which groups end up mingling with one another? Because if they ever did, how come we never saw any Waterbenders native to EK (and I don't mean subgroups that live in isolation like the Swamp Tribes, I mean Waterbenders of EK culture).
Hell, even with the AN being all monastic and shit if there was any contact there should still be a few airbender bloodlines belonging to other nations.
I've read stuff published post Korra that talk about there being more interactions between people from different nations, like how the rise of Kiyoshi reveals her mother was a runaway AN nun. But in what respects to the two shows canon, it seems that things like that never happened, looks and bending capacities seem to be locked to their geographic location.
In fact, going only with the shows, the actual first real contact between groups was the 100 year war, which ironically puts the FN and its superiority complex as the pioneers in racemixing due to the colonies.
So what's up with all that? Was Sokka really the first Water Tribe teen to ever see an Earth Kingdom girl and think "Oh boy do I want to irrigate that fertile soil"?