How much difference is there, really, between various European nationalities? They are all white and Christian, using similar language and script, mostly the difference is in the Christian denomination.
About 1000x more than the difference between US states
Similar language is a stretch. Sure, European languages have similar roots, if you go enough far back. But the ability to speak your native language with a fellow European of another nationality, and have a fluent conversation, is quite limited.
I guess you could ask the same question of the middle eastern countries. Pretty reductive perspective, no?