It’s long been the case that Google gives enhanced ranking positions to websites which use video. Facebook promotes posts and content which uses video FAR more than any other kind of content. Your chance of being on the first page of a Google search are 53x greater if your homepage has a video on it.
At every turn, you’ve seen that using video is the only way you’re likely to have any kind of impact or have adverts which convert at anything like a reasonable cost – so why doesn’t it work?
The new ability to use video as a Facebook Page Cover Image is being rolled out gradually, not all pages are currently able to use video, and some users have reported still images in place of video when using mobile devices.