Because there is no UN mandate to do so
You wouldn't get one either, irrespective of what China or the US thiink. It's not the UN's job to forcibly invade a country because its leadership have steamrollered it into bankruptcy even though there is political violence as part of it. An example I can think of is the Solomon Islands where the SI government appealed for foreign assistance to retake control of the country. The Australians and New Zealanders did, and they are still there.