(ii) the probability of a landing spot at a program which pays less (or nothing at all)

In normal employee terms, you'd only leave your current job if the salary and conditions at the next job were good enough to make it worth your while to relocate (moving is a major hassle). You also wouldn't quit your current job unless you were reasonably certain that that you could land a better job.

For a lot of these kids, they have a bird in the hand. Moving across the country isn't worth it unless they're sure that they'll get more money, get more exposure, land in a better situation, etc.