> west coast [...] South China Sea

Or to use Occam's Razor: There's a greater density of assets to control, nearer at hand.

Yep that's correct. And the main asset that navies protect are trade routes. So you can see that they would want to protect the South China Sea since that's their gateway to the world. But the West Coast really only connects to America, who wouldn't trade with them anyway if they took over the West Coast. So it would be totally counter-productive to send ships there.

China can be many things, 'insecure' is not one of them. Maybe try 'patient'.

Edit : From a Chinese perspective, the Pax Americana has been a blip in history.

Edit : Downvotes are fine, but would like to hear the arguments against the facts of the region.

China hasn't been a single govt or people, they have had very different ones over time, of course many different regional languages over time before the 20th centry. They aren't one single culture than went x thousand years. Of course they do have an ancient and proud culture with many achievements and led the world at various times but they weren't just one group. Just like there were dozens maybe 100s of different political entities and countries and philosophies going from Greeks to Rome to modern America and European countries. There wasn't one continuous 'people' for more than 2k years there either.