Similarly, immigrant groups tend not to integrate, critics from foreign countries really are usually misunderstanding a large number of important points, and so on.
Between any two cultures, there is a huge amount of overlap. But nothing quite coincides, and the differences end up mattering a lot.
Your 12th century stories are interesting and all but do not reflect the reality of the world we currently live in. If you live in a large city and have a medium-to-high salary, which is a reasonable assumption to make given HN's demographics, it is in fact easy to get acquainted to other cultures by traveling often (possibly living abroad), learning other languages and/or befriending (or dating) foreigners. Much easier than used to be a couple centuries ago, in any case. If you do, you will find that deep down beneath the veneer of culture humans are very much the same. I don't have data to back this up, this is only from my mere experience.
Pretending that cultural differences don't exist is harmful and condescending; acting as though they were insurmountable and irreconcilable is downright dangerous and leads to disaster.