I think a lot of the times it's because it's easier to try and categorize people into little boxes instead of actually listening and evaluating their arguments. But if you actually can have a real conversation with people then you find that no one is really all conservative, or liberal or whatever.
Growing up in the family I did, we always loved to talk about politics, religion, guns and abortion, lol.
Though the first time my wife met my family when we were leaving she asked us why we were all fighting, I said we weren't fighting just having a normal discussion. Just we all get excited and talk loud and wave our hands around :)