Who said they were smart? I've known employers who gave job applicants polygraph tests and used handwriting analysis to help decide who to hire. Ouija boards are just as reliable.
The hiring process is the single most stressful and time consuming duty of a manager. An employer will grab at any straw that promises to make the job a little easier. The old saying, "It's who you know," is the Golden Rule. When it comes to hiring, a manager is desperate for any reason to favor one person over another, because there is seldom any significant difference in any of them, and usually not many from which to choose.
Credit checks are used for screening future employees because it is easy. Every person's report is laid out in the same format. When a manager is asked why he hired a person, he/she can point to a box on a form and say, "He had a high number."