Unity allows easy spreading of disease and corruption, cancer cannot spread between different people. If a union in one country is corrupt, it won't automatically affect the unions in another country.
China was a united entity against the mongol invasion, didn't work out very well, because you take out the center, the rest collapses.
Europe was deeply divided, and just because your neighbouring castle surrendered, doesn't mean you will. Because you are the local noble who owns the land, rather than some bureaucrat employed by a distant emperor.
I am sorry, is this like troll logic?
The entire history of war is large and powerfull empires bullying smaller ones.
This is so silly I am not even sure how you can argue this. Like did USA fight world war 2 as 50 seperate armies, one for every state?
That is not the case. You have period of imperial blobbing, and also period of intense fracturing. Europe fractured after Rome, because smaller polities were much easier to maintain, with fewer layers of bureaucracy to feed. The small Frank kingdom (that only owned half of France) was able to fend off the great Jihad from the Umayyads that had crushed the Visigothic Spain. Whereas if you took out Rome, or the Chang-An of China, the rest of the empire would be paralyzed and unable to act.