Social sciences may be behind though as you say, I do not know as I'm not in that field.
That matters for a few reasons:
(1) the average person more frequently encounters psychological, social and medical issues more than they do math problems. And since the research in those fields tends to be pay walled, people are at the mercy of things like SEO spam medical and health sites.
(2) wrong ideas in medicine or psychology can (and have in the past) damaged entire generations of people. So in that sense their blast radius can be very large. This means that peer review is especially important and that there's a potential negative externality to posting preprints and drafts before they're finalized. I suspect we'd have to solve the peer review and quality problem before STEM-style preprint archives become the norms in all fields.
Much of the medical and life sciences space does not publish on Arxiv or OA platforms.
It's slowly changing with Green OA initiatives being pushed by government donors, but not there yet.
> Social sciences may be behind though as you say
Econ, a lot of PoliSci, Finance/Business, and Computational Linguistics was very early on the OA/Working Paper movement.