T.L.
I think it's because if someone is already employed, their talents/skill set are said to be more valuable since they are working. They look more attractive to employers who are looking to hire. Let's say that the candidate is working, but not in their chosen field. They are still more attractive to the employers because they are working as opposed to sitting home doing nothing. For those people who are unemployed and looking for work, but haven't been able to find a job, the sediment that I got when I once worked was that if you have been looking for work for a long time and have not found a job, you are either too picky or really don't want to work. So, you don't look as "attractive" as a candidate who is already employed and looking to either get in their chosen field or advance their skill set.