Also, you can't just apply for a clearance out of the blue. You typically need a sponsor and a need to know. If someone is just a candidate they don't have a need to know and therefore wouldn't even be able to get the process started.
Good point. This is true.
Is it? The background investigation is separate from actually holding a security clearance. The clearance is sponsored by the site's security manager, and depends on, among other things, a successful background check, and the need to know, but the investigation can be performed in anticipation of a future need.
Ours were started in Missiles-go-boom 'A' school, where our need to know never exceeded Confidential, but we were training for Above Top Secret jobs down the line... Even those of us that failed out of A or C school, sometimes on the basis of the investigation.
As for sponsor, maybe that'd be an additional filter on candidates, finding someone in State, Justice, Defense who will sponsor an investigation. It could be telling just to know that 'no agency wants to touch' this individual with a ten foot application.'
And the cost could be shouldered by the campaign. The ability to come up with the scratch for an investigation, would also filter out random jackasses filing their candidacy as a publicity stunt, or a dare...