I remember when JPEG was a new thing and people decompressing their GIF files to recompress as JPEG despite the two having very different lossiness patterns.
People are weird about image compression. Codinghorror has several posts about improving image compression but HTML and javascript size optimization is largely ignored. "What if Discourse didn't send two copies of every post on forum page loads? Nah, too complicated."
https://blog.codinghorror.com/zopfli-optimization-literally-free-bandwidth/