Why it happens?
If the damage occurred to the drive’s electronics, it most likely could be fixed. If the damage occurred to the, for example, system areas of the disk, leaving the data zone intact, those data could be theoretically, and (in some cases) practically, recovered by a professional. But, if the data zone is damaged, the data in the damaged area will not be recovered since the loss of data has already taken place. In this case, one can only rely on the back-up.
Two major reasons
For example, hard drives are capable of handling operational shock up to 300 G, their electronics or mechanical components still could be damaged by the vibration or by an impact. Liquid spilled on the drive may short electric connections or even penetrate inside the drive, especially inside the drives with removable media (Zip, CD-everything, Tape drives, etc.).
All media, drives and storage system components undergo thorough testing, but in spite of this, statistics guarantees a few bad drives among hundreds of good ones… You may be unlucky and buy one of those. But even in this case, data back-up and a good manufacturer’s warranty can make losses partially or completely recoverable.