Because the idiot hasn't been there - as I explained. I don't know why you'd ever defend an absentee parent (male or female) who willingly chooses to have nothing to do with their child.
I'm not defending anyone, but I have to say, simply not being there doesn't automatically cause a child to hate an absentee parent. I know this from personal experience.
My older son didn't know his biological father, our marriage broke up when he was an infant and he moved away and never saw him. I remarried when my son was 2 and he started using his stepfather's name (although we didn't change it legally). When my son was 6 or 7, I told him about his real father, just so he would know if we had to use his legal last name for anything.
Beyond that, we didn't talk about him, and he never even asked me anything about him.
He never once expressed any hatred for the man, he was simply a non-entity to him and not relevant to his life.
Once, when he was in his teens, a friend of my son's was having some major issues with her dad and my son made the comment that he was kind of glad he didn't know his father, because sometimes dads seemed like more trouble than they were worth! But that's the only thing he ever said about him.
So I don't know if any of us should speak in absolutes - while your nephew may have decided, for whatever reason, that he hates this man that he doesn't even know, not everyone feels or reacts in the same way.