To be fair, it’s also basically impossible to have extremely high quality AV1 video, which is what a lot of P2P groups strive for. A lot of effort has gone into trying to do so and results weren’t good enough compared to x264, so it’s been ignored. AV1 is great at compression efficiency, but it can’t make fully transparent encodes (i.e., indistinguishable from the source). It might be different with AV2, though again even if it’s possible it may be ignored because of compatibility instead; groups still use DTS-HD MA over the objectively superior FLAC codec for surround sound because of hardware compatibility to this day. (1.0/2.0 channels they use FLAC because players support that usually)

As for HEVC/x265, it too is not as good as x264 at very high quality encoding, so it’s also ignored when possible. Basically the breakdown is that 4k encoding uses x265 in order to store HDR and because the big block efficiency of x265 is good enough to compress further than the source material. x264 wouldn’t be used for 4k encoding even if it could store HDR because its compression efficiency is so bad at higher resolutions that to have any sort of quality encode it would end up bigger than the source material. Many people don’t even bother with 4k x265 encodes and just collect the full disc/remuxes instead, because they dislike x265’s encoder quality and don’t deem the size efficiency worth its picture quality impact (pretty picky people here, and I’m not really in that camp).

For 1080p, x265 is only used when you want to have HDR in a 1080p package, because again x265’s picture quality can’t match x264, but most people deem HDR a bigger advantage. x264 is still the tool of choice for non-HDR 1080p encodes, and that’s not a culture thing, that’s just a quality thing. When you get down into public P2P or random encoding groups it’s anything goes, and x265 1080p encodes get a lot more common because x265 efficiency is pretty great compared to x264, but the very top-end quality just can’t match x264 in the hands of an experienced encoder, so those encoding groups only use x265 when they have to.

Edit: All that to say, we can’t entirely blame old-head culture or hardware compatibility for the unpopularity of newer formats. I think the home media collector usecase is actually a complete outlier in terms of what these formats are actually being developed for. WEB-DL content favors HEVC and AV1 because it’s very efficient and displays a “good enough” quality picture for their viewers. Physical Blu-Rays don’t have to worry about HDD space or bandwidth and just pump the bitrate insane on HEVC so that the picture quality looks great. For the record, VVC/x266 is already on the shortlist for being junk for the usecases described above (x266 is too new to fully judge), so I wouldn’t hold my breath for AV2 either. If you’re okay with non-transparency, I’d just stick with HEVC WEB-DLs or try to find good encoding groups that target a more opinionated quality:size ratio (some do actually use AV1!). Rules of thumb for WEB-DL quality are here, though it will always vary on a title-by-title basis.