Even in a large department, individual specialties may have one or only a few experts. Sometimes none if the department is looking to branch out or fill a gap. There's also typically competition between groups for hires, so no group gets the final decision on which candidate to present (to the department, then the dean, and on up the chain...)

As for the filtering, there aren't any widely accepted ways for a nonspecialist to rank candidates. Hiring committees are a standard part of a professor's job.