Some of this depends on who you think the readership will be. Those scientific societies provide a sort of floor on the quality of what they publish, which allows non-experts like me to read the papers with some level of trust in the results.
Or, a layer of indirection away, it gives science journalists a source of stuff to write about that's at least somewhat validated. I do worry a bit about what happens to science journalism, which is already generally of poor quality, when they start reading random junk on arXiv.
Of course, there are alternatives where the papers could still be free. The government could fund the journals, arXiv could implement do their own peer review system with quality indicators. Probably more I'm not thinking of off the top of my head.
I have no first-hand experience, but my intuiting would be that a lot of the gatekeeping these societies do is focused on reputations, organizational hierarchies, and funding considerations. Which I would not really consider productive, especially in light of the abysmal state of reproducibility and the apparent lack of incentives to tighten things up.
I don't think "cannot" should be the standard. If the same or better standard of peer review isn't provided (empirically) by some other system, then that's a legitimate criticism of the other system.
There are certainly other ways of getting to the same thing, but what I think a peer review process in isolation lacks is a sense of trustworthiness of the peer review.
I.e. as a layperson, I'm not particularly qualified to gauge the quality of an arbitrary paper. I am implicitly not qualified to gauge the quality of peer reviews, because I can't gauge the paper itself.
I can defer to the opinion of experts, though. These societies unify the diffuse expertise of who knows how many people. I.e. I don't work in sociology, so I have no idea who is a reputable sociologist, but I have some degree of confidence that the peer reviewers they choose aren't total quacks.
It's a useful heuristic for non-experts. I don't know anyone or anything in rocket science, but I can defer to what NASA says. I'm not a marine biologist, but I can defer to the NOAA. Etc, etc. There's value in that heuristic, even if it's not perfect.
> I have no first-hand experience, but my intuiting would be that a lot of the gatekeeping these societies do is focused on reputations, organizational hierarchies, and funding considerations.
This is indirectly useful to my thinking. The society is focused on funding and income, which is largely predicated on them being trustworthy (theoretically), which gives them an incentive to provide quality peer reviews. I would imagine a society with 0 standards loses subscribers pretty quickly.
arXiv could certainly do the same thing, but my impression is that arXiv is more focused on making papers available than being an authoritative entity on anything. "This paper is on arXiv" doesn't imply the same level of trust that being in a journal does. They derive value from being open rather than from being trustworthy.
arXiv definitely has value as well, I don't mean to imply that it doesn't or is bad in some way. It just has different goals, from my limited knowledge. It's likely invaluable to people in their field, especially for those qualified to gauge the trustworthiness of an individual study.
arXiv could definitely build some kind of trust system like that if they were interested, but I really think their primary audience is experts who don't have a need for a trustworthiness score simple enough for a layman.
That's not totally obvious without sorting out the details. Similar to copyright, it seems to be inefficient (in the sense that it artificially restricts supply) but certainly a lot of people think it helps improve the overall supply of creative works.