Using filesystem images in a bundle seems unnecessarily complicated when we have zip, tar, rpm, deb, and a host of others. Images often contain all sorts of junk that just isn't needed and can even be harmful. Conflicting UIDs on files anyone? And the image types that would probably be recommended are the compressed write-once images which are not any different from a file archive.
Plus, it is getting hard enough to read the output of mount already. Why make it even more complicated by having every installed app mount more images?
Another thing is user modifications. Editing filesystem images is quite annoying, as I know from mucking about with initramfs images. The user might like to fix a bug in an app's Python script, or change a URL to use a different search engine. A read-only compressed image needs to be unpacked to its own directory, modified and then unmounted, rebuilt and remounted. Sometimes weird problems show up because it was packed as root but repacked as a user account or extended attributes or the SUID bits got lost.