That would certainly have been the argument at first. Battery life is EVERYTHING.
But Netflix announced they would be using it in 2016. YouTube started using in 2018, the year the standard hit 1.0.
Apple had plenty of warning and time to get the necessary hardware acceleration blocks available by now. Chips have multi-year timelines but it’s been 4+ years.
Now on the other side, Netflix actually started using it in early 2020/late 2021, depending on device. YouTube rolled it out wide in 2020. Twitch is working on it.
So Apple is at most 2 years behind the content. It’s not like it’s been 5+. And if they decided to wait to see how those early rollouts went due to hardware timelines it might make sense support only appears to be coming now (based on someone pointing out AV1 shows up now in the AVPlayer framework documentation).