Well, the other liberal arts departments don't portend to not be liberal arts departments. Economics sells itself as rigorous and predictive. And "multinational capture" has occurred in myriad actual scientific fields, for similar reasons (using the prestige of rigorous, academic work to convince governments to do things).

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.