Because for decades since the opening of China, they have been overfishing their (already big) EEC without regard for sustainability.

China is literally raking the ocean bottom with large nets destroying everything down there be it fish related or not, and therefore preventing marine life from reproducing in its natural environment that doesn't exist anymore. No other country is doing that, most use large nets but don't go all the way down where most ecosystems exist, they usually stop the net several hundred meters above the bottom.

There is no more fishes around China's EEC, including the South China sea, it's all been depleted by the Chinese themselves. So China is desperately entering other countries EECs to steal their fishes. Chinese fishing fleets have been caught around Africa, all the way to South America, in waters that belong to other countries according to the UNCLOS convention.

Countries that have the means are fighting back. The Chinese don't dare going to Japan's waters anymore because their fishermen get caught most of the time there, and then their fishing boats get shot down by coast guards, each sunken boat is a lot of money lost for the Chinese. Same thing with Korea, Western nations and countries with more aggressive policies such as India, Brazil or Indonesia.

But African countries and most of South America don't have the means to catch and stop Chinese fishing fleets, well except Somali pirates. In most cases the Chinese come and go unchallenged, taking as much fish they can and depleting resources for the local fishermen who manages to sustain the supply for centuries. It sucks but it's just the old story of the strong taking from the weak, the same old story.