Hackers exploit social engineering, which avoids technical security systems, by manipulating the psychology and behavior of a human being. Social engineering techniques, such as baiting emails or pretexting phone calls, manipulate victims into providing confidential information or performing actions that impede security details.  Attackers find it cheap and easy, as they need less specialist knowledge to make expensive use of trust, curiosity, or fear to trick their target.

Source: GBHackers