A large part of Europe was a single "country", namely the Roman Empire. What is currently China is arguably China + Tibet + Xinjiang + parts of Mongolia + part

A large part of Europe was a single "country", namely the Roman Empire. What is currently China is arguably China + Tibet + Xinjiang + parts of Mongolia + parts of Korea + parts of whatever Heilongjiang/Jilin/Manchuria should be called. China (including China proper) has occasionally broken up into multiple polities. One could also argue that the European Union kinda, sorta constitutes a single "country". One that would become an instant superpower if we who live there wanted it to be... But, on the whole, Europe tends to be more fragmented than China. I'd say the geography has a lot to do with it. China has a couple of big rivers that are well-suited for long-distance travel. Europe doesn't. Europe is also a peninsula of peninsulas, often with mountains (Pyrenees, Alps) going across where one peninsula is attached to the next.