Building on them accomplishes nothing legally. They get a 500m safety zone and no territorial sea limit. The Chinese build on them for tactical military reasons.
Technically the PCA ruling on the Philippine claim is void for lack of jurisdiction. It's an ex parte arbitration. When it went ahead ultra vires and ruled on the merits, it resorted to some bogus equity standard in determining whether Taiping Island was an island or a rock. It's definitely an island capable of sustaining a community. Therefore it gives rise to a 200nm EEZ centered on the island. The PCA decided without a factual foundation that it was a rock. Thereby by legal jiggery pokery defined away a territorial issue, it admittedly has no jurisdiction over. Rocks do not give rise to a 200nm radius EEZ, islands do. Woody Island in the Paracels is also an island under the UNCLOS criteria, therefore it too gives rise to a 200nm EEZ radius. Then there is the 200 nm radius from Hainan Island, and the Chinese continental shelf. No matter how you look at it China has a huge EEZ claim to the SCS. Other states do have conflicting claims, the competing claims are competing EEZ claims. There is regional agreement mechanism to negotiate these disputed claims which preempts UNCLOS jurisdiction.
EEZ claims have nothing to do with freedom of navigation. FONOPS are a phony issue that the US uses to bootstrap itself into the maritime EEZ claims, in which it has no legal interest whatever under international law.