This has been discussed previously, and some uploaders have expressed themselves contrary to that. There are two main concerns:
1. Books are sometimes updated with corrections, better formatting, new stories in "omnibus" editions, etc. Publishing a large collection would perpetuate old versions of updated files.
2. Often the uploader writes some background text on the book and author, or on the technical details of the ebook, which are probably of interest to potential downloaders.
This, of course, applies only to what should or shouldn't be "officially" published at MobileRead. Nothing prevents anyone to download everything and upload the collection to his/her website or to P2P networks.