Occasionally the editors -- inevitably based in Chennai -- spot a typo that the reviewers missed. Sometimes they cock things up massively, especially equations -- I had a big argument about a tickz-based scheme once. The formatting is done automatically and for me as a latex author I really think we could do that very easily. Colleagues who use word see the text transfer to something better as a major value add. The publicity the journal adds is a strong function of how "good" it is. Science and Nature effectively exist because they are Science and Nature.
Frankly, they profit from: academics writing articles (funded by someone else); academics editing a journal deciding who should review articles (usually, but not always, for free); academics reviewing articles (always for free); and academics citing articles in subsequent ones.
The whole system is a house of cards, and a relatively immutable one.
Right, the perception of being associated with authoritative and knowledgeable publishers which follow certain formalities (in other words "the proof you as a scientist belong to the Club") has been an important part of academic career progression, even if in the last few years the reputation of peer-reviewed papers in general has taken a dive.
What if we could look at authors, peer reviewers and papers as a graph of weighted edges to come up with a score that was independent on journals as a concept? And where there is an ontology for the semantics of edges (not only number of citations)?
However, I got the (personal) impression, that this is only well established in fundamental research (which typically comes with few economic interest). As soon as the research is not paid by the state but by private companies (such as in medicine, robotics or any other "applied sciences"), scientists have a hard time to choose a OA journal (i.e. either it does not exist or you are not allowed to publish there). Changing this scheme is of course quite difficult, since too many commercial parties still benefit from it (which likely can only be change by law)...
It's acting as an impartial rating/filtering service for the non-scientist administrators.
In essence, the funding agencies want a way to evaluate scientists and institutions without asking their scientists and institutions to do so, probably because they don't trust them, and also because they have very strong incentives to avoid making any subjective judgment themselves but instead defer to some "objective" outside source - so they use paper counts published in "proper places" or the existence of papers published in "very good places" as the evaluation metric to circumvent the (genuinely very hard!) problem of evaluating the quality/quantity of the actual research done.
And so this incentive, attached to much of the money flowing within academia, trickles down to evaluation of people when hiring and promoting (the committees also often look at paper counts and publication venue rankings instead of trying to evaluate the actual papers - which is time-consuming, and if the papers are not in "your field", then very hard to make an informed judgement) and so to the individual motivations of almost all the people in the system, who have to take into account the "proper publishing rituals" or severely limit their career.
So having worked a bit with administration and evaluation of funding proposals I kind of see why something like that is valuable in general, the main problem being is that the costs are enormous and not really commensurate with the provided value - however, all the costs and barriers are suffered by "someone else" (i.e. the scientific community), while all this value is provided exactly to the funding decision makers who have the power to prevent replacing the current system of Elsevier&co, but don't really suffer from its problems. And when I say "funding decision makers" I don't mean people who have the money and personally care about if it's spent efficiently, I mean all the administrators and bureaucrats running the process of allocating someone else's money or hiring scientists in e.g. some public institution, and whatever "sticks and carrots" these administrators have in their career. This means that to outcompete the current process, any solution would have to benefit them (not the scientists) or it won't be accepted and used, and it's hard to imagine what that would be since there's a huge barrier of entry (e.g. it must work for evaluating/ranking all scientists/institutions, across all disciplines and across all the decades of previous work, or it's not useful) and a huge inertia, as the criteria are also included in very many hard to change legal documents e.g. contracts for long-term funding projects, bylaws and processes of many organizations and committees, actual laws regulating funding institutions, etc; if we had a clear winning replacement launched today, it would still take at least 5-10 years to switch to it.
IMHO the way to change is not a competing solution - it requires institutional change, with the major decision makers simply choosing to make decisions on different factors that do not include the metrics of Elsevier et al. Here's a talk by Stonebraker which gives a strong related argument - https://www.youtube.com/watch?v=DJFKl_5JTnA&t=1220s . However, this institutional change is not that likely because, as I said, for the people who can change this there's little incentive to change, and the people who would benefit from this change are not in a position to make it.