Because JPEG just works. It is not the best by far, but everybody supports it, and it is usually not worth saving a few bytes in exchange for worse support, add

Because JPEG just works. It is not the best by far, but everybody supports it, and it is usually not worth saving a few bytes in exchange for worse support, added complexity and extra processing. Lossy compressed images are usually not the most significant consumers of bandwidth and disk space. Videos are. That's why there have been a lot more focus on video formats than anything else, there is a lot to gain here, not so much with still images. JPEG-XL is super-complicated because it supports plenty of things most people don't really need. Webp is somewhat better supported because it is backed by Google, it is also what is essentially a single frame video, so if you did the hard work on video (where it matters), you get images almost for free, and it saves Google a tiny bit of bandwidth, and "a tiny bit" is huge at Google scale. We are seeing the same thing with audio. MP3 (1991) is still extremely popular, the rest is mostly M4A/AAC (2001). We pretty much have had the perfect audio format now, which is Opus (2012) and yet, we don't even use it that much, because the others are good enough for what we make of them.