Because China has interests that are in opposition to US interests, and they can sway US opinion any way they want by covertly manipulating the feed. They absolutely can do significant harm with this, including but not limited to selecting politicians, inciting chaos and political unrest, and even economic destabilization. I'm not sure that the US government actually has a much higher potential to do you harm than a foreign enemy of the US with a weapon like social media as you stated. You could make a strong argument that the political shitshow we are currently in is partially due to foreign interference through social media, and that is before they owned the actual platforms. The US government is not incentivized to destabilize itself at least.