their deployments are by sea. weapons are unloaded and vehicles based at known island ports and airfields, with little cover. There's only so much foreign base warehouse space for permanent storage, typically they'll stack munitions before the war starts. Meanwhile PLA deployments are inside a large country with massive amounts of cover. the reserves are in country.
to track PLA weapons deployment they'd have to monitor the entire country, to track USN aviation and USAF weapons deployments PLA only need to watch a much smaller area of forward airfields and ports.
Due to lack of cover, to track USN movements PLA can use satellites and long range drones, but thanks to cover and noise, to track mobile ground forces you have to be right on top of them. But they can't get right on top of them, they'll have to overfly much of China to do so. There's a reason why ranges for anti-vehicle munitions are in the 10 km range, rather than 200 km range of anti-ship munitions. It's much harder to see ground forces, including TELs, from far away.
A conventional first strike is unlikely to work. only problem would be a nuclear first strike, but that's why China needs a massive strategic arsenal so a nuclear first strike is off the table.
Historical comparison: it took 6 months of Operation Desert Shield for US to build up enough forces to even attack Saddam Hussein Iraq, a country much smaller than, much weaker than, and with much less geographic cover than, China. China is not going to watch US build up in Guam or Okinawa for 6 months and be surprised when an attack happens.
