Recruiters don't participate in their client's daily activities - like, sitting in on project meetings, marketing strategy meetings, So, they don't really have the insight necessary to pick one applicant over another.
They have to use a more general approach, which leaves them vulnerable to candidates who look great on paper and interview well, but don't suit the the job.
So says Olivier Blanchard.
Once, after being hired, the recruiter who interviewed Olivier, told him that he had been the weakest of fifteen candidates.
According to Olivier, that didn't reflect the poor selection ability of the hiring manager but the lack of knowlege of the recruiter who didn't know enough about the job to recognize a good fit.