"PhDs issued" grows exponentially (since each professor can issue PhDs to multiple students), but "job offers" grows only linearly.
This supply and demand imbalance tilts the power balance almost 100% in favor of the professor in any interaction with grad students. Professors have acquired essentially unlimited and arbitrary power to dictate conditions, and grad students have no choice but to comply or leave academia -- throwing away a lifetime of work and preparation.
More at http://www.nature.com/nbt/journal/v31/n10/full/nbt.2706.html
[1] http://www.nature.com/nbt/journal/v31/n10/images/nbt.2706-F1...