Why does Trump antagonize China and not just do what we need to do to fix unfair trade?

Because antagonizing China is something Trump can do via his Twitter feed.

Actually renegotiating trade policy requires the cooperation of China, Congress, and the Corporatist Kleptocracy which currently benefits from "free" trade. That's much harder that Tweeting.

He can't yet actually do  very much.  

fuq china...trump wants to make usa  great again