That's exactly what's happening with Open Access Journals: they charge the authors to finance editing and other editorial processes, and distribute papers for free and with liberal licenses.
It's not the cost of a value-adding service, but the cost of having a huge technical debt that nobody has bothered paying down on.
I misread your "I never understand this argument." as not understanding what consp wrote, "this" was a bit ambiguous, and I originally thought you meant "I'm the scientist, let me do science instead of communicating clearly what I did and observed" but now I see "this" referred to the argument consp quoted. So we are both agreeing with him. Add to that that I was confusing a sibling comment with what consp wrote. :)
At least one publisher, despite claiming to 'accept LaTeX', appears to just copy-and-paste from the pdf output into Adobe InDesign and manages to introduce tonnes of new errors, which then I have to have substantial time to fix (for free, again). [And usually I don't manage to catch everything.]
Why exclude those fields? I would argue that poor editing skills are fairly widespread, regardless of domain.
Copyediting is still badly needed, but that's typically foisted upon a grad student; the journals don't contribute much as far as I can tell.
Actually some conferences come with "publishing fees" in the form of "if you want to publish at our conf of course you'll have to present here, which means you'll buy a ticket, which is veeeeery expensive".