For the same reason 23rd/24rd century humans in Star Trek largely share the (often antiquated, even by today's standards) opinions, mores, mannerism, (slightly modified) fashions and language of humans from the 1960s/1980sWhy would Romulan and Vulcan have anything in common??? Because a couple of millennia before they lived on the same planet? I mean we live on the same planet as the Japanese. How much do English and Japanese sound alike???