Why do people say we need to be nice to others when they are not nice to us?

The world is literally a survival of the fittest. Why do people try to say everyone should be nice and help each other out when we all know that is not how it works. Everyone tries to get the hell past each other and be the better one. To live in a world where you have to be nice to everyone just to feel a bit better about your life is just a waste of a life. I’m living proof of that.