The answer is probably the same as for why not AV2 everything; a lot of hardware couldn't support it today. But in 10 years?
It seems we're running up against fundamental limits of human-engineered video codecs at this point. There might be a lesson in there.
And it's not really hardware hitting limits, it's specifically software decoding on somewhat weaker machines.
Adding custom hardware like tensor cores to the stack would serves a different use case.
What I'm saying is the performance problem is a "code smell". The algorithms are getting so complicated that perhaps we are approaching fundamental limits of heuristiccs; we might get better + faster results ditching "smart" algorithms and just learning the codec in a much higher dimensional space.
Again specialized hardware, but a different approach to it.
This could work / works for video too, give it lower resolution / quality images and AI upscale. Its predecessor would calculate intermediate frames for example.