I have some guess as to why there isn't much adoption of av1 by the torrent release groups and sorry for the lack of spell check and double checking fact since it's late for me.

1. They don't care about av1.
2. Speed: it takes longer to encode a av1 video than either of the other codecs and also add the fact that they might not have GPU hardware acceleration for av1. Nvida just added it to there four thousand series.(note: I'm aware that GPU hardware acceleration give worse results than only using CPU.)
3. Limited device support. I sometimes throw a movie on a USB drive into a TV that does support anything but h264.
4. Unfamiliar with av1 so they would won't put the time and effort to find the sweet spot they like (don't fix what isn't broken). And also add the fact storage was and might be getting cheaper so av1 seems less necessary. (Note: av1 main point was to save bandwidth. My guess is pricay video streamers will enjoy it more than the torrent groups.)
5. They might dislike av1's compression artifacts. I gotten use to h264 and h265, sometimes when I watch an av1 encoded video It takes me out of the Immersion but that can be fixed with time.
6. Lack of guides. (Not entirely sure but could be a factor.)
7. Might had a previous bad experience with av1 and haven't looked back.
8. There might but less newer torrent releases group. (This seems unrelated but I have notice that they usually are the first to adopt new codecs.)

Note: I like av1 but I haven't found it outstanding for anime other than crazy low bit rates and from what I remember it is great with high resolution video which anime isn't know for.

I don't re-encode my rips but I might use it for transcoding for Jellyfin when ever set that up.