simply put companies (even Wal Mart) have a vested interest in hiring people who will stay as long as possible. Recruiting is expensive and time consuming. No one wants to do it twice.
While this is the correct answer, I still got a laugh out of the Catbert, Evil HR Director rant earlier in the thread! If you work for or have ever worked for a large corporation, the Dilbert comic strip is daily gold...
My wife likes to bring up the fact she beat out multiple Phd's for a research job at UIHC in pediatric endocrinology. Her primary function is to recruit family members of Type 1 diabetic children to participate in various test studies. She's great at it because we have two children with Type 1, so she believes in the work and has true empathy for the families of the recently diagnosed kids. She didn't have better credentials than the Phd's who applied, but they were just looking for a stepping stone while she'll retire from there. Easy choice...