In my experience what you say rings true; the job descriptions are such that its hard for a single person to hit all the reqs and if the company is underpaying, it's going to be hard filling that position.

With regard to internal hires, yes, the jobs are posted to meet some HR criteria and are merely a formality and thus get filled quickly.

But.... This is in tech which accounts for a minute part of all jobs opps, so what applies to tech may not necessarily apply to non tech jobs.