Because most people are relatively honest and they naively assume others are the same. Since most live in their own personal bubble, nothing of any great import usually touches them in any meaningful way, so they exist under a false sense of security. Their good intentions are then preyed upon by the predatory element of our society.
Also, the act of carrying a gun doesn't make you a gun fighting bad ass, any more than the participation in a forum like this does. Most of the time, it just makes you a D-bag with a gun.