How long do you think five thousand troops would last in LA? They wouldn't make it to West Hollywood before they were all shot up by every cop, gangbanger, Korean grocer, and lunatic there. What if once ship got caught, or was boarded? They'd have to execute it perfectly, knowing that once the jig was up, the US Navy would sink everything floating with a Chinese flag within about 12 hours. No resupply for you, comrade.
Aside from being a great way to piss us off, turn every liberal stronghold in the US against them, and waste a huge amount of material, troops, and ships in the process, there's no reason for them to try.