Because it's completely naive and stupid to assume that someone who is "nice" is also honest and fair, or that someone who is honest and fair has to also be a "nice person." If you ever watched the mini-series, Lonesome Dove, or read any of Larry McMurtry's novels, there is a specific character by the name of Woodrow F. Call who is a classic example of someone who is honest and fair but not a very "nice" person. In fact, he can be downright dis-likable. On the other hand, we have Bill Cosby... someone who had all of America fooled. Turns out, he wasn't the "nice guy" everyone thought.
So? Honest and fair are usually attributes of a nice person. That is just a fact.
And I am saying they are unrelated and that is a terribly naive viewpoint.
Well, I disagree. I think they are very closely related from my own experiences. Perhaps yours have been different. What now?

I guess we will have to agree to disagree on that subject.
Doesn't matter about your experiences. Appearing to be nice means
nothing with regard to honesty and fairness. I can think of a million examples of completely despicable people who fooled others into believing they were nice people. Ted Bundy and John Wayne Gacy, for example.
Yes, I guess I've had different experiences... I've been screwed over by "nice people" too many times to count... I go to by tires for the car, the salesman is a nice guy, I pay 25% more for my tires because he was so nice. Happens to people all the time.
By the same token, 90-year-old farmer I knew in North Carolina, grumpy and cantankerous as the day was long, totally unpleasant to be around, rude and obnoxious to others, no hesitation to hurt your feelings with what he said, didn't give a shit if you liked him or not... but the man would give you the shirt off his back to help you. When he passed away, I went to his funeral and was shocked... you couldn't get into the building where his body rested for viewing. There were thousands of people there to say goodbye to this very "un-nice" man. Come to find out, he had helped so many people through the years and never mentioned it. The man was honest, perhaps TOO honest at times... The man was fair... but he wasn't NICE.