Because to have a true hegemony, you need to be able to project power around the globe mostly uncontested. This is very difficult for China as they are essentia

Because to have a true hegemony, you need to be able to project power around the globe mostly uncontested. This is very difficult for China as they are essentially boxed in by Japan, South Korea, Vietnam and the nations of the South China Sea, even if they do take Taiwan. So they will never have fully uncontested access to the sea and thus to the world, even if they do surpass the US in military power abroad (which is doubtful).