If you recognize that, in certain terms, believing certain things has positive instrumental results even if they're not true, why can't you simply abolish the false beliefs and just create those results directly?

Human brains are (loosely speaking) Universal Turing Machines - they can emulate any computation. So if we're looking for a particular set of results, we're not tied to a way to reach them that's invalid. There's always a valid path that gets us to where we want to be.