Because Moon wants to harken back to the days when Korea was a tributary state to China. He even visited China and got treated like shit and still said, "Oh, that's just our usual relationship."
And he's just going with the popular idiot Korean consensus that, "Hurr durr, America bad. China will help us reunify and become Empire of Korea." Our current administration is just as much as a fucking dumpster fire as the one we threw out.
We should be allying with Japan or something to counteract China's rise, but we aren't because of historical shit. Surprise, surprise, we were just as fucking bad in Vietnam as the Japanese were in WWII. And as much as I hate to say it, we're modernized because Japan force-fed us modernization by tossing out our archaic and ancient monarchy. The only caveat here is that they raped and killed a fuckton of Koreans in the process, which is where the hate stems from.