I don't know about your "millions of dollars" figure, but academic journal publishing is a for-profit endeavor. Platforms like IGI, Manuscript Central, etc., do not exist to provide a service. They exist to make money for their parent corporation and investors.
There are some journals that are self-published by editors or a university at little cost, but the most prestigious journals contract with a publishing platform to manage submissions, reviews, approvals, and publication.
Very common to pay the “managing editor” (non-academician employee of the publisher) a salary, but many (most) reviewers and editorial board members perform their role gratis as part our scholarly service to their profession.
While some journals pay peer reviewers for this work, to many of us purists, that feels “dirty”; when reviewing for such journals, it is common to direct the stipend go to one’s institution/department.