My guess is that it is often hard for people to grasp that HDDs loose value much faster than other items they own. New HDDs are larger and offer better price per TB, and older HDDs have a higher risk to fail.
I can buy new HDDs at 16€/TB, why should I spend 12€/TB on a used disk?
One of the best offers I could find is 300€ for 20 TB, which makes exactly 15€/TB.