Why does anybody see China as a threat?

Because China is the only country that can have a political system that unifies a population of 1.4 billion. No political system in human history can unify a civilization of such a colossal population. The Chinese political system has already unified China for two thousand years, while the EU has yet to struggle with it.

Those who view China as a threat believe in the theory of western Real Politik. The view of the west of international politics is based on power. For thousands of years, Europe politics has been hegemony or imperialism. That's the political structure China used before 720 BC. After 700 BC, China invented another international political structure called Proper Governance. For two thousand years, China holds Confucius's doctrine of Proper Governance as the optimal political structure. Hegemony and imperialism were outdated in China for two thousand years already, however, hegemony and imperialism are still the norms for the West. The West never conceive the idea of Proper Governance. They speculated the behavior of China according to European history.

Western culture is the monotheist culture whose idea of government is based on fear or terrorism. They view China as a threat based on their own behaviors.