civilian shipbuilding and military shipbuilding are complementary industries. China has both, the US only has one (military)

And even their military shipbuilding industry is decaying, showing that indeed you need both.

Having both allows greater innovation and technological development as people and resources can move between both. What you may learn in civilian shipbuilding will have applications in military shipbuilding

It's probably a significant factor that recently US Navy is eating the constant stream of L's