Now try to save a gif on Twitter without resorting to scripts or developer tools. Try to right click and save an Imgur gifv, then upload that back to imgur again.
GIFs are simple. You save an image file. You're done. Video is complex. You can't just have an mp4 (well, x264) file because that's patent-troubled. You can't just have a webm file because that's not supported by Apple devices.
Even these live photos are difficult. You need to add Apple JavaScript to your page, and the live photos themselves are made up of two files:
>Your Live Photo will be stored as a pair of files: a JPG file and a MOV file.
Yeah, GIF itself is a bad format, but simplicity is why it's still around.