Gov't red tape... Job is typically advertised for a while. They need to wait until the open period is done. Collect all resumes. Then evaluate and rank them, then apply veterans preference/other special categories.
Then HRO sends the list of the top 1-5 or so candidates to the person trying to hire. Then they arrange interviews, decide who they want to hire. Or decide they all are not a good fit, and readvertise again.
Then there is a ton of paperwork and a job offer gets sent. Then the paperwork REALLY starts!
Well, yeah, basically. Also, an agency or department will have a limited number of openings, determined by its budget. A particular manager may want very much to hire you, but lack a "hiring point" to do so.
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