Why would you not set your home alarm system when you are home?

I have been seeing an increase in home invasions while people are at home. Many are the homes in suburban areas with the false security that nothing will happen to them. They have alarm systems, but may only arm them when they are away and not while they are home. Also, there have been cases of children beng kidnapped from their homes by intruder entering an unlocked door or window. Why would you not set your alarm??