Modern body armor is made in two parts the plate that is the actual armor and the carrier that is a harness that holds the plates. The plates tend to change every few years well the carriers tend to last longer.
It should be noted that just because they are wearing a plate carrier doesn't mean that they are wearing armor. The plates can be dummies which are used in training because they are cheaper and the real ballistic plates can be damaged.
Only the stripped down plate carriers favoured by special forces only have ballistic plates for protection.
Most general purpose body armour have a lot of integrated Kevlar, with hard plates acting as additional reinforcement for the chest and back.
The plates are easy enough to replace or switch with training dummies, but the Kevlar panels also degrade with time, and are far more difficult and costly to replace/simulate.
As others have said, China has an adequate stock of body armour it could issue at short notice in the event of sudden conflict. In addition, Chinese companies make and export a lot of modern body armour, so the capacity is there to meet big PLA orders fairly quickly.
Because of that, and the fact that China is not at war with anyone, it makes little sense to spend so much money on things that will, in all likelihood, never be used before they expire.
Chinese troops could just carry extra weights in their packs to simulate the additional weight of armour.
As time passes, and China's existing stores of armour date expires, you will probably see more and more PLA troops being issued with expired armour to train with regularly. But give the size of the PLA standing troop size and the modest number of sets the PLA is currently buying, it will be a long time before training with armour becomes common practice for the PLA.