system
Given the following post, respond as if you're commenting on Reddit:
human
[AVATAR] Is there a reason most benders have a corresponding eye color to their element?
gpt
We see in LoK that the nations got their start living in isolated cities on top of their respective lion turtles. Wan didn't even know other lion turtles existed, and most people were too wary of the spirit wilds to have ventured far on their own. If they were isolated for long enough, it's entirely possible that each distinct population center would have evolved certain genetic traits unique to them (like you mentioned, eye color).

This persists into the Avatar world, even before Aang. Travel would've been a lot more difficult back then (except for perhaps the Air Nomads, but they were always small population-wise and likely didn't interbreed too much). It's only once the indrustrial revolution started by the Fire Nation began, did travelling between nations become significantly easier, and intermingling much more common.