What they don't realize is that in aggregate, their willingness to do it for free jeopardizes their ability to do it for money in the future. It's a nice little game theory problem. A single defector from the "only work for money" strategy probably benefits; the rookie journalist gets his foot in the door, gets the "exposure" and the industry connections, and so yields an advantage over his peers.
But once all his peers defect from the paid strategy, the whole thing comes crashing down. None of them achieve any advantage over any other. They just drive the median compensation down towards zero.
The claim that there's an abundance of high quality journalism is hogwash. There isn't. There's a glut of no-talent regurgitators. Most of them have poor knowledge of the subject matter, don't bother checking facts, don't do any original research, and generally offer little of value.