Or the close neighborhood of the Philippines, a major US ally with a mutual defense pact. Or Taiwan, a major US ally and trading partner. Or Vietnam, a growing trading partner. There are ongoing disputes between and among those groups.
A huge amount of shipping goes through the South China Sea, including most oil for S. Korea, Japan and the US West Coast. Meanwhile, goods bound for Europe and the Suez from those places also go through there. Something like 10% of the worlds shipping. Letting China just call "dibs" on international waters and set up a chokehold would be weird to allow.
I don't think anyone is urging war with China. But stopping their advance in it's tracks.
Meanwhile, China has been aggressively expanding its influence worldwide, in South America, Africa and Afghanistan.