Why is it so difficult to get your foot in the door at most International Organizations?

At first glance, it can be difficult to understand why it is so hard to get your first job overseas. After all, there is a huge amount of need out there, aid organizations are always stretched (and usually recruiting). You are bright, educated, enthusiastic, have low salary expectations, and are ready to go – why don’t they want to hire you? In fact, why do most of them not even want you as a volunteer?