Why?
In addition China wants to rule the world.
What better timing will they ever have with the U.S. crippled by so many different issues, such as a weak president, a deeply divided nation, and the worldwide pandemic in play? So they start attacking smaller nations within their immediate realm to seize control of that part of the world.
Not all countries are going to be as easy to capture as the U.S. in its current state. They don't have to go after the U.S. directly, they can just sit back on that front and let the events unfold for the country ti destroy itself.