Most Helpful Guy
Easy to pickup and play. Other genres of game are more technical (fighting games have advanced stick/pad motions + button combinations, each RPG has a completely different battle system, abilities list & items to learn, sports games have advanced control features in game that make regulars WAY more advantaged to win vs casuals).
Shooters are walk, look, shoot, throw frag, aim down sight, change weapon. 8 year olds pick up FPS games really fast. Have that 8 years old kid try doing that with Diablo 3, Madden or Tekken Tag Tournament. Poor kid will get frustrated just trying to learn the basics!
Also, guns have always had a machismo appeal to them as well.
Most Helpful Girl
I personally just find them to be an amazing stress & aggression reliever :P