Pirates don't care about codec licences the same way they don't care about the copyright of the content they pirate. Therefore they will always pick the codec which is easiest to target, has hardware for encoding and decoding readily available. Most either watch content on PCs, smart TVs, nvidia shield / firestick and almost none of those have av1 hardware support (maybe PCs but only recently).
Pirates also work directly with video files. Each file has one bitrate and one codec, and needs to work as universally as possible. There are sometimes two or more versions of the same video available that use different bitrates or codecs, but each added version has a cost in clutter (they typically show up as separate entries in the UI), disk space (relatively expensive when it’s people’s personal disks) and P2P swarm availability (critical if using P2P), so you won’t see too many. In contrast, just about any streaming service will have several different encodes for each video, automatically selecting one based on bandwidth and codec availability. That makes it relatively easy to adopt new codecs, since users who can’t decide them can just get a different encode.
However, pirates do care about file size and quality, as demonstrated by the adoption of 10-bit H.265. So AV1 should be coming, eventually.
