In the case of nepotism, the advantage comes from accident of birth. So many people are convinced that their relatives are better or have an ulterior motive for getting them hired that you can’t trust them.
Contacts are different, in that you don’t get someone to vouch for you unless you do good work, as in Bricker’s example. Anyone in a position to hand out any kind of job has had people try to use you as a contact where you say, “I’ll see what I can do,” and then dump the resume. In any case a contact just lets you get your foot in the door - you still have to interview. I doubt relatives really have to interview.