RAR is a proprietary format... there is no free-software code to decompress RAR archives (the license agreement for the code that is available prohibits using the code to 'reverse engineer' the RAR format itself, and I'm guessing most projects don't want to have to change their license agreement to a non-free-software license to be able to include RAR support).
Not sure why anyone actually uses RAR these days...I really don't get it.