The value that recruiters offer is delivering good candidates. If you convince a recruiter that you're a good candidate, then they will run around selling you to companies! And companies will listen, because the recruiter has build a reputation by delivering good candidates in the past - that's how recruiting works.

But if you just pay an agent to run around selling you, there's no longer any value being added. Why would companies listen to a recruiter that just refers any old candidate that pays them?