They're suited for delivery because they're based on video codecs (WebP on VP8, AVIF on AV1) for which consumer hardware typically has hardware decoding support, but were not designed for still images. AVIF is newer and substantially better than WebP, but still has limited colour depth and channels, so it's comparatively poor for editing. Lossless mode is not good for non-photo content. Encoding is relatively slow without hardware support.