Because the left and the media has driven that into people's heads. Not to mention that the Democrats want to conveniently have their connection with the war, and it's aftermath, totally wiped from history. How convenient. The majority of the men who fought for the Confederacy never owned a slave in their lives, and a large number of the Confederate officers served longer in the U.S. Army than they ever did in the Confederate Army. I hate it when people want to destroy someone else's history to promote their own history, and their own politics. I was born and raised in New York State, but I'm a history buff who studied The Civil War from both sides for many years, and traveled all over the south visiting the battlefields where good men from both sides gave their lives. They're not just destroying Confederate monuments, they're destroying everybody else's too. The St. Gaudens monument on the Boston Common, dedicated to Colonel Robert Gould Shaw and the black soldiers in the 54th Massachusetts Infantry was defaced. This isn't about racism, it's about anarchy and insurrection. They're just using race as an excuse to justify their destruction.
I doubt any of the assclowns who defaced the 54th Mass monument could tell you who and what it was about.