* France - France (Overseas region and department),
* Russia - Russia (annexed region),
* USA - USA (commonwealth),
* China (People's Republic of) - China (Republic of)
Goes to show that the notion of "nation" has fuzzy borders.
Those routes are busy simply because there are a number of Egyptian expats working in KSA who travel back home for vacations and/or family events and stuff. Apart from that, Cairo and Riyadh are as international as you can get apart from the language - they speak different dialects of Arabic, they have different views on religion, they have extremely different cultures, and they call each other's food shit.
Both parts (Arabia and Egypt) were ruled by their respective dynasties for almost a half millennia, far longer than Ottoman rule.
I don't know why there's a lot of traffic between Seoul and Tokyo, but it's certainly not because the former was briefly ruled from the latter. Comparing these examples to the Russian and French examples is laughable.
Or that these aren't nations but empires pretending to be nations.