Just like the continents weren't always separated, I don't think we can remember when it wasn't like this. It's not hard to conquer and destroy an egalitarian society. They would simply let go. To the victor go the spoils. And they write the record so we can't collectively access those memories.
The avatars you mentioned are the ones who return to remind us of our higher self. Some humans remember and operate at higher vibration. We can no longer see their existence. If you activate your highest self - you will experience nirvana and will be able to operate between the dimensions. Or remain with those avatars.