Here's why it works...
Most companies can't recognize talent from a resume and need someone to tell them someone is talent.
Recruiters do a pre-screen interview and get an idea of whether you can pass an interview, recruiters then tell employers someone is talented, and here is this great persons's resume. Since the recruiter has already checked that the individual has a pulse and can answer questions intelligently they usually get the job.
Most companies could insource their recruiting efforts if they were willing to do more than just throw resumes out, if they paid their HR department 3 months salary for everyone who was hired (like recruiters are) there'd be drastically different attitudes and practices in HR.
Instead HR just throws out resumes that don't match the specs the manager sent because there's no incentive to actually hire people.
It's not uncommon for someone to apply for a job and a month or two later a recruiter calls you for the exact same job detailing how the company can't find anyone to fill the role, oddly enough they then hire you based on the interview the recruiter was able to leverage through their relationship.