Animations should just be videos. No reason for an animated image format, an animated image is just a video with no audio track.
Or let me rephrase, why is WebP as a animated picture / video format insufficient?
Then, there's optim being made in WebP to allow fast jump to keyframe, even when there's transparency. Video codec don't allow that, and you can have an arbitrary long torture sequence of transparent frame that needs to be decoded back when the video comes in the view again.
Last, animation are usually low-fps (~10fps): there, video codec don't perform very well and are basically keyframes. So the difference isn't as great as one would think.
Oh, and hardware need a 'reset' between decoding tasks, to reconfigure memory, and decoding can't be parallelized.
<video autoplay muted> says the same thing.