Dropbox quietly replaces the insides of JPGs with a better compression system (lossless change) to save their storage costs, and then returns it to normal when the file is read. (It may be they do a JPEG XL transform, for all I can remember about the article which explained it).

I wouldn't be too surprised if every big CDN already does something like that internally as a competitive advantage, and if so then the incentives switch around and there's no benefit publically switching to JPEG XL - when everyone has it, there's no easy way to do better than everyone else.