Are there trust issues with allowing third-party MP4 embeds into user-submitted content such as forums/hosted blogs? Do we need a "no sound"/"max file size" attribute on the video tag?
Hence why there is a push for alternatives to Gif/APNG and mp4/webm. We want a video-like animation, that plays on page load.
Now, the real question is, why don't mp4s play when we hit play?
I can only speculate, but I think it's because mp4s are streamable media, where a troll can simply decide to keep streaming mp4s and drain the user's bandwidth. (Never mind the fact that it is also a problem with Gifs.)
Not all of us want that. In fact many of us would rather decide for ourselves whether or not to play an animation, sound or not.