Random access is completely possible within a zip, to the degree that it's needed for cbz; you might not be able to randomly access within a file, if for some reason the cbz was stored with deflate on a jpeg, but you can always access individual files independently of each other, so seeking to a random page is O(1).

ZIP literally has a central directory.

I don’t understand what’s the point of any of this over a minimal subset of PDF (one image per page).