The incentive structure just doesn't seem designed well. It creates a zero sum game. When downloading you can either:

1. Not seed to 100%. This damages your ratio

2. Seed to exactly 100%. In terms of ratio maintenance across all seeders this option makes the most sense

3. Seed past 100%. You build up your own ratio but deny other downloaders from reaching 100% which hurts their ratio. They must spend longer seeding the torrent to reach 100%, which further decreases the likelihood of subsequent downloaders from reaching 100% when seeding

When you seed past 100%, you essentially have to rely on bad actors to create more upload work for good actors. If there are no bad actors then seeding past 100% is to the detriment of other good actors, who you want to protect because you also rely on them for system health. And private trackers aim to minimize the number of bad actors.