How do we know if people are hard to find because people are actually hard to find, or people doing the hiring are terrible at their job? Partly both, but also

How do we know if people are hard to find because people are actually hard to find, or people doing the hiring are terrible at their job? Partly both, but also missing a qualified applicant often costs far less than making a bad hire. The ratio of decent hires to bad hires needs to be as high as possible. In comparison, the percentage of qualified applicants thrown out almost doesn't matter (unless it gets to something truly absurd like 90%). (Really it does matter, just not compared to making bad hires. Interviews and delays are still expensive, just less so than failing to get new employees up to speed.) Interviewers that make bad hires are terrible. Interviewers that turn away good hires are merely bad.