It could be that there are a lot of images/versions that simply never get seen, or seen rarely enough that it's inefficient to store them. And they're being cached by the CDN anyway, so popular images will still only be generated once per edge.
bingo! Extra storage for all the resized images that never get seen, lack of flexibility to introduce new sizes, plus requirements for watermarking and attribution on images are the main reasons why we are moving away from pre-conversion to dynamic image resizing
