Oh, yeah, it definitely resets the compression context between frames. But so does a video format.
No format that I'm aware of supports lossless compression that spans frames. Which leads to a funny situation where some images compress best by custom outputting a .gif or .png with no compression, and then zipping it.
As I understand it all video compression uses delta compression between frames. And you mention another issue I forgot, that it can't reuse Huffman codes or any common segments between frames. Since they are independently compressed.
No format that I'm aware of supports lossless compression that spans frames. Which leads to a funny situation where some images compress best by custom outputting a .gif or .png with no compression, and then zipping it.