First I hope you do know we do use all of our brain but that we only use a bit of it every second or so. Second to answer your question I believe it is because people can't get over our basic instinctions such territorial instincts. What I mean by that is that we want to stay normal to who, where, or what we are. Such that when people fight over land it is because they are used to the idea that it is theirs or if someone fights over there belief it is because they think they are right and don't want anyone to change that.