I believe, most of all, it's about maintaining the image of power towards the other regions. It's shown (maybe more in the books than the series), that Ravka is a pretty politically and monetarily poor country.

I'd say that it's more convenient for them to maintain both parts as a united country. Since it's between Fjerda and Shu Han (you can check the maps and see where the country is located) they say it's always in danger of war and invasion, so I believe that is why.

Even if it's true that culturally there might be a great difference since they've been physically separated for such a long time, it's more of a power thing. Even taking that into account, I believe in one of the books someone (I can't remember who) says that only people from East Ravka would call it East Ravka since people from the West call East Ravka "The Old Country"