Many reasons Nev.
The main one is them not wanting to get involved in the selling process and having to deal with all aspects of it, from simply cleaning the records, researching them, listing them, selling them, posting them and then dealing with the customer if not happy. The process takes time if you're not used to doing it and many people don't have that time to spare, or want the hassle. I take a lot of time doing this for people, I don't just get the records and slam them on a list, I have them for a couple of weeks making preparations before they get listed. But it's a full time job for me,which I love, of course.