There are simpler reasons that I've observed internally. I've seen excellent resumés go completely ignored simply because the manager wants to wait forever for a unicorn with 10 years of highly specialized experience but is somehow magically just starting out.

Also make job postings just so they can whine to the government later about how they can't find workers.

In larger companies, they also do it when company policy requires them to advertise externally for positions even though the hiring manager already knows exactly which internal candidate he intends to promote to it.

No serious investor is dumb enough to be swayed by data that's easy to fake. Public companies are required to publish their financial reports every quarter.

It also gives them the chance to scoop good candidates (highly skilled or willing to be underpaid).

It also gives them an idea of the quality of candidate available at that wage, potentially firing an existing employee to hire a similar one for less.

I was reading elsewhere that some companies who want to transition an H1-B worker to a permanent worker are required to post the position first. They never intend to hire anyone other than the experienced H1-B worker, so the ad stays up for a period of time until they can pretend that the H1-B worker is the only qualified candidate. I don’t know how true this is, but it sure sounds plausible.

There is a company in my area that has been "trying" to fill a role in my field for at least 6 years, non-stop. They spend money every month to put up a fake help wanted as on indeed.

Salary is a scam to get people to work unpaid overtime and never really getting to clock off. It's effectively making an employee always on-call.

Honestly, I don't think it matters how much you get paid, if you are expected to work beyond 40 hours a week, you deserve extra compensation for doing so.