Preservation doesn't always mean to preserve every raw bit or analog signal off the original medium, but preserve the content itself, even if compression is used, because certain niche movies never get re-released on modern medium, you're just left with the original Laser-Disk or VHS, maybe even DVD if you're lucky, and that's it, so compressed video is better than no video because you have no legal way of obtaining that content today if it's not sold on any physical medium or on any streaming service.

Compression is needed on torrent sites because few peers are interesting in downloading and seeding a 60+GB raw file of a movie if the 12GB mkv looks just as good on the 1080p Wallmart monitor.

There are of course also RAW rips of blu-rays out there but mostly for AAA IPs like Star Wars, LotR, Matrix, etc, not for niche content that have a reduced audience.