Because the two countries do not share definitions on what that even means. They both have to agree and both need a set procedure for adjudicating such disputes for it to be real. If the US says it's "IP theft" and China disagrees, there is a dispute on the legality of the act in question.
So for example, in France, there are many acts which are trademark infringement in France that are not trademark infringement in the US. A Frenchman can accuse an American of trademark infringement for an act that is not trademark infringement in the United States, but is infringement in France. They can bring a lawsuit in France, win, and potentially enforce that judgment on assets in the US with the cooperation of an American court despite the fact that the American did not, by the definitions of American law, infringe on anyone's intellectual property.
There are no such cooperative arrangements between the US and China despite recent attempts to set them up. There are also only limited agreements on what is and what isn't permitted.