The read failures are also attributed to other parts of the system, which for the end user still end up in failed reads. The author links to a sales PDF from Quantum.
e.g. the robot dies, the drive dies, the cartridge dies, the library bends, the humidity was too much.. multiplied by each library, robot, drive and cartridge your data is spread across.
Or, a fun little anecdote, the cleaner had access to the server room and turned off the AC of the server room, most disk drives failed, and the tapes melted inside the robots.
e.g. the robot dies, the drive dies, the cartridge dies, the library bends, the humidity was too much.. multiplied by each library, robot, drive and cartridge your data is spread across.