China has already notified everyone that they will not abide by the ruling of the court. They didnt even send counsel to some of the preliminary hearings.
It wasnt until jurisdiction was established that they actually started appearing and arguing against the petition by the Phillippines.
If China used its might diplomatically to work on a joint treaty with the nations involved, which usually entails opening their checkbook, they might get more cooperation.
But in this case, if they have to pay, they want to call all the shots too.
So by placing themselves on these artificial islands. They improve their seat at the table before hand.