[derf_]

My understanding (now several years out of date) is that Arm decided not to pursue licensing a design, because every customer they talked to had requirements that were so different that they would have essentially been one-offs that each required significant additional engineering. I cannot speak to the others you mention.

I believe the team we worked with at Arm during AV1 standardization is no longer there, which is too bad. They were really great guys to work with.

Your suspicion is mostly correct, though obviously you cannot share too much of the DCTs as these must be bit-exact and are different for each of the standards. But especially things like the compressed tile cache for reference frames used in motion compensation are extremely complicated (to save memory bandwidth and power) and entirely shareable. The SRAM used for line buffers is also a lot of area and shareable. And so on.