If piracy is moral and ethical and enables us to share knowledge, why do private trackers gatekeep this knowledge? It goes against the principles of piracy. Do they do it just to feel superior about bring in a sekrit club?
I would say it is more of a practical consideration. Private trackers generally enforce upload/download ratios. This ensures the health of the sharing pool stays good.
Usually torrents remain seeded because private tracker users are encouraged to seed everything forever. In addition, often if a private tracker has a bonus system they will offer extra bonus points for seeding low-seed torrents, and some even automatically mark torrents as freeleech if they're below ~5 seeds, encouraging people to revive its seed count in a targeted manner.