Its because they're young. Youth is not a time when you are rational; trust me I've been through that stage. When you lack a sense of failure, when you're in the environment you're in as a kid, everything looks crystal clear. Most people realize the world isn't once they leave high school and head off to other things. They find that people are as varied as they are, that not every republican is an asshole and not every christian hates your guts. They also, for better or for worse, develop their own set of stereotypes to use daybyday. Open-mindedness is something that can be relatively static and it should be something people strive for. The problem is when people use it for posturing. Being open minded about new experiences helps to enrich your life and creates some of your best memories. That's not something to show off, its something to live. Maybe I'm just being closed minded though.