China always united by several dynasties

This is false, even in a limited sense of counting only "China proper". It was talked about here. And I've gone into a bit of detail in the /r/badhistory thread here.

If you take a look at the timeline I made for that thread here you can see even just China proper had multiple incredibly long periods of division.

However, as I said in the previous /r/Askhistorians thread above, while Chinese unification is vastly overstated, it's still more unified than Europe, especially in the last 700 years or so. I hope a proper expert can go into why this is the case, as all I have are theories.