Have you ever published a paper in an academic venue?
What publishers offered was delivering paper versions of articles to your local research library. Now they only host PDFs online. Apart from that their added-value work is negligible. The biggest improvement I've seen by a publisher of one of my papers was changing "Sec." to "Sect." when I was referencing section, and putting page number in blue rather than black (true story).
Anyway nowadays when a paper is accessible directly from a public repository, it is that version which will be used most often, and it includes literally zero work from the publisher.
All the real work is done by the authors, and by the reviewers, who are other researchers doing their normal job (i.e., they're not paid specifically to review papers, and more importantly not paid by publishers).
The only thing that publishers provide is reputation/prestige, and that's only because of inertia, and it's difficult to change because they own the titles of journals they publish. That's one of the numerous reasons why bibliometrics is an entirely bad idea and is counter-productive for science.
Also, not all open access journal are charging author, that is a lie installed by publishers lobbying (and this is how they actually spend money). You can research "diamond open access" or "overlay journal" for some examples of economical models where journal are high quality, open access and free to submit to.