I think the difficulty stems from several factors. One, knowledge of the field is very new and it has not been part of project process for most companies. They have established ways of thinking and the duties that are now a UX designers used to be divvied out among other positions. BAs covered some of it, DEVs covered some of it, and it was all kind of a hodge- podge. So perhaps it is understandable if they look at a UX designer and say to themselves, "Oh...they work like a BA/dev/QA person." Depending on how it is described to them.
Secondly, UX itself is so multidisciplinary and we have many professionals who started out in other fields. So it's entirely possible that a UX designers has dev skills, or BA skills, or designer skills. But not EVERY UX designer will have these backgrounds.
Thirdly, for all that UX champions simplicity, it is difficult to describe to people. I find myself having to explain to UX to just about everyone in my company, and it's quite difficult. By this point I've refined it fairly well, but I definitely see why it's hard to explain the discipline without someone saying "Oh, so it's like "
Add to that the fact that most recruiters don't really look in-depth at any one position. They tend to use a net rather than a fishing line to snag people and they don't take the time to educate themselves on individual disciplines.
Anyway, that's my two (three?) cents.