Applicants don't self-filter because they need a job. They like to eat, they like to wear clothes, they like to sleep indoors. They often have other people in their lives that also like to do those things and rely on the income of the applicant to do so. They also aren't very able to pool risk. Most people have between one and two wageearners paying towards "household" expenses. There's not a lot of redundancy here. Contrast that with the average amount of employees at an employer. When you're hiring, having a position vacant for weeks or months is something you can work around much, much easier than you can work around being unemployed for weeks or months.