Sometimes there is no 'why'. We just learn how each language does it and go from there. French doesn't use the 'un/une' for jobs either. If you think that's bad though you should see the Scottish Gaelic - 'S e an tidsear a th'ann' - He is a teacher - literally, it is the teacher that is in him.
(edited. The teacher not a teacher that is in him... doh)