Essentially, because it is reversible.<p>In other words, the information needed to extract the true raw-text password is actually included in the encrypted string.<p>With a hash, only a representation of the raw-text is being stored. Even if you knew exactly what was done to create that representation, you're still a long way off from understanding the raw-text.<p>If you treat both the key and the salt as being "known to the developer only" and either of these becomes compromised,  you still don't have they raw-text of a hash, but you've got everything you need for the encrypted.