Most private trackers require that you log into their website after x amount of time (usually it's something like at least once every 90 days but you check the tracker's own rules).
In fact most trackers usually don't bother emailing you to remind you of this rule and will simply disable your account for inactivity.
Many sites also waive it for seeding torrents 24/7.
Not sure if it's "many" but I'll take your word for it. I double-checked the trackers I'm at and only 1 of them counts seeding as site/tracker activity. Seems pretty rare for private trackers to allow an account to exist without logging in indefinitely.