Rather than "why are there so few job postings?", ask "why are there any at all?" Rationale for this pivot is here:

I took a business law class, and my professor all but told us that each job posting is all but an invitation for infinite liability for an org, as the legal system is setup such that any applicant may accuse the employer of discriminating against them and the employer has to prove that they didn't. Accordingly, I wonder why we have job postings at all, and so to connect it back to your point: it could be worse, it's weird that it isn't, and the way it is is likely due to design.