First, there is the Chinese whispers from within the company. They don't seem keen on finding candidates directly, which in this field is as easy as it gets, so off they go to the recruitment agency. My (now former) boss writes a job spec, this goes to HR (who aren't familiar with any of the technical details) and HR add some company blurb and send it on to a recruiter, but not before crippling the job spec by slapping on a below-market salary that nobody competent will accept to try and save some cash.
The recruiters think they have plenty of candidates who will take the salary, but the candidates simply aren't good enough or don't have the right experience. The recruiters have no way of telling this, so they keep telling the company that there are plenty of great candidates and to keep interviewing.
From the applicant side, several friends of mine who are generally looking to move to a new job had this role aggressively pitched to them by a recruiter. Most of them would be brilliant for this job. What my friends really need is a ten minute phone call with my old boss to see if it's a potential fit before starting any formal interview process. I'd have been happy to put them in touch directly at an earlier stage, but there has now been contact through a recruiter and I don't want to meddle behind the scenes.
None of my friends ended up going for the job since the recruiters were telling them confusing things, and all got better offers than the advertised range elsewhere. The company have unsuccessfully interviewed a few candidates that the recruiters have pushed on them, the team has been desperate for someone new for months (and they're crucial to the company's success) and people like my old boss have no idea and little way of knowing how close they are to finding the right people.
And yes, the stereotype of the shiny-suited young "failed salesman" recruiter is unfortunately true in my experience.