Which means that when economists have theories that fly in the face of what people believe (like pretty much all of science), people get more annoyed.
To me (like you), that suggests that GP is telling a just-so story about why we can ignore economists.
There's also the matter of the "rational actor" theory of human behavior, the a priori assumption of classical economics. It describes a being which is basically non-existent among flesh and blood humans. But corporations can form themselves in the image of this mythical economically rational being. And so, if classical economics does objectively describes the world, corporations are the uber-mensches of such a world and poised to dominate it in perpetuity - a clear incentive to capture and control the field, lest dissident economists succeed in espousing a revision of their core assumptions.
There are also interesting differences in how economists view themselves compared to other humanities, see this (biased) paper: Fourcade et al, the Superiority of Economists http://pubs.aeaweb.org/doi/pdfplus/10.1257/jep.29.1.89 Economics tend to be a more insular and hierarchical discipline, and views itself as non-normative.