(Depending on company culture) There is a lot of risk in making a bad hire, because in many companies it's very hard to fire someone for poor performance or being difficult to work with. In those situations, it's safer to leave a position vacant instead of making a bad hire.

One workaround is to hire contractors, or contract-to-hire. It does put risk on candidates who are leaving other jobs, though. (I won't contract-to-hire when I'm leaving a job, but I will if I'm unemployed.)