And this is because that is what people have been taught. Information that undermines this view is simply not taught. Have you ever heard of the Corwin Amendment?
http://www.wbur.org/cognoscenti/2013/02/18/the-other-13th-richard-albert
“The modern justification for all of the bad things that were done in the Civil War is slavery”
This view of the Civil Was is just as incomplete/incorrect as the “lost cause” tenet that slavery had absolutely nothing to do with secession or the coming of war.