That's interesting. Brotli has wide browser support although its less than 5 years old but webp is reaching a decade and Safari still doesn't support it...
WebP has an excellent lossless image compressor (like PNG just 25-40 % more dense), but the lossy format has weaknesses that people focused on, and slowed down the adoption. The initial lossy encoder had weaknesses in quality -- it had bugs or was a port of a video coder. Nowadays, the quality is much better, but the format forces YUV420 coding (does not allow YUV444 coding) which limits the quality of colors and fine textures.
> but webp is reaching a decade and Safari still doesn't support it...
That’s a philosophical objection. For a long while Mozilla also was of the opinion that WebP is not “better enough” than JPEG/PNG to warrant the addition of another image format which the entire web must support forever using only one available implementation.
Plus I think there are still some unresolved patent claims on the VP8/9 video codec (which are the basis for WebP).