It's being worked on. There's a lot of overhead inherent in random access, which really isn't necessary for most applications. ;)
http://en.wikipedia.org/wiki/Linear_Tape_File_System
A Random Access Filesystem that runs on LTO5. (And it even supports OSX!) Cool hack. Just don't ask me why they bothered.
http://www.quantum.com/Products/TapeLibraries/index.aspx
http://www.dell.com/us/en/enterprise/storage/tapebackup_auto...
For reference -- an LTO5 Tape holds about 3TB of data. (and has decent compression -- so that could go much higher)