AFAICT AV1 is just a significantly more complex byte stream to create vs h264, especially if you want that byte stream to be meaningfully more compact than h265 created with x265 @ slow. x265 @ slow was already pretty time consuming[1][2]. And x265 itself had a hard time against x264 for years too[3]. So it may not be that the AV1 initiative is neglecting the public encoders and more that the math and algorithms for computing an AV1 byte stream really are that difficult.
[2] I ran a test 1080P encodes using x264 and x265 slow both CRF 20 via handbrake on my m1 MacBook Air and got ~20FPS, and ~4.4FPS for x264 and x265 slow respectively. On my i7-1165G7 NUC I ran the same test but directly invoked FFmpeg and got ~16FPS and ~4FPS for x264 and x265 slow respectively.
[1] https://mattgadient.com/x264-vs-x265-vs-vp8-vs-vp9-examples/ (this is x265 medium, but it has a nice gui)
[2] I ran a test 1080P encodes using x264 and x265 slow both CRF 20 via handbrake on my m1 MacBook Air and got ~20FPS, and ~4.4FPS for x264 and x265 slow respectively. On my i7-1165G7 NUC I ran the same test but directly invoked FFmpeg and got ~16FPS and ~4FPS for x264 and x265 slow respectively.
[3] This is just a short debate on reddit as an example https://www.reddit.com/r/DataHoarder/comments/lx41cx/x264_vs...