They aren't. But meanness is still a viable evolutionary trait. So the small percentage of people who are mean, or who act in mean ways, can achieve great success in relative terms (though usually forgotten by history except in extreme cases).
If you really stop and think about it, day to day, person to person, from the guy/gal that lets you cut into line in your car, to the person who just says hello or nods at you - you encounter far more kind people than mean people in your life.
The better question is; Why don't you remember the people who aren't mean?
The answer to that, is complex and simple all at once: You, like most people, have learned due to terrible outdated methods of child raring and/or religion - to hate yourself. So when someone is mean you think "they're probably right." But when someone is nice you think "they didn't really mean it." and you forget about it.