- Many websites support embedding images in user content but not videos (Gmail, Hangouts, and Stack Overflow, to name a few).

- A "minimum viable animated thing" tag is more complicated for videos:

    <video autoplay loop muted src=video.mp4>

- Supporting videos in img tags and sending the right Accept header means that image hosting sites and CDNs can immediately start serving video files to browsers that support them, without the page needing to change. This applies to every hotlinked gif out there.