This is curious as post docs were sorta training positions in ancient times, picking up additional skills to make an academic or commercial hire more likely. For example, do a synthetic PhD and p-doc in a natural products group. Or mix computational and lab groups. Hence stipends and eased tax rules.

If it’s just a job and not training, then the profs can reject more candidates if they wish. People in jobs are expected to hit the ground and start running sooner, which might keep people from getting a slot. Still have to publish to show progress, and skimping on that will hurt future prospects.

Bottom line, if you want a “job” get out of academia.