2D image compression is missing a dimension. Much of the savings to be had in a video compression algorithm are going to be based upon the similarity between frames (ie. across time) rather than within a given frame. Therefore, an algorithm that is designed only for the 2D image is not going to deliver the goods when applied to video. An example would be animated GIF. It's terrible.

You can of course just have a sequence of individually compressed frames, just as you can do with PNG, TARGA, or JPEG, but these are not usually the ways you would want to distribute the video due to the huge sizes.