There is a lot of truth in that the best jobs are sometimes not listed, or listed simply because it is company policy. When I played music full time back in the day, networking was key to landing paying gigs. It is true that for every well-known pro musician that there are 1,000 more out there sitting on park benches who are just as good if not better. The difference is that the ones on the park benches are not known by people who can make a difference.
When an opening comes up in a working band, especially one that works a lot and makes good money, rarely are there open auditions for that open spot. The truth is that a working band is far too busy to audition 10's of 100's of candidates to find the perfect match. Working bands rely on networking and word of mouth to find the musician who is the perfect fit. There is competition in the music world but there is also cooperation. So let's say that working band "Band X" who had gigs booked for the next 2 years, has a big following, and makes great money (for musicians that is ) has their guitar player quit and now needs to find a replacement. The chances are that Band X is well aware of other working guitar players out there and has a list of ones that have earned respect. Often the replacement is found in another working band that has not quite achieved the level of success as Band X. For a guy like this to be offered a job by Band X would be a huge boost to his or her career. Other times the replacement guitarist might be someone who the departing guitarist recommends especially if the departing guitarist is leaving on good terms. An example of never burning bridges.
The same principle works in some areas of nursing. Say there is an opening in a highly desired area. The replacement candidate may well be someone who the hiring manager already knows, or even a recommendation from the nurse leaving that position. Rarely do they have a cattle call for awesome gigs. The reason being is that it is way too time-consuming and rarely yields the quality results as well as highly recommended candidates.
Never under estimate the power of a personal recommendation.