The short answer to this question is that, because most people do not read academic histories, and our history textbooks have not always engaged critically with issues of race, that depictions of slavery in novels and films like Gone With the Wind are the only ones that many Americans see.
The longer answer is that these romanticized depictions of the plantation South were fundamental to a reshaping of American memory, especially Southern memory, of the antebellum period, the Civil War, and Reconstruction. Known as the Lost Cause by historians, this view in some ways recapitulated antebellum defenses of slavery. The antebellum South was remembered as a place of genteel manners, happy slaves, and good masters. The brutality of the institution was simply effaced. These memories became widely held, not coincidentally, during the post-Reconstruction period, a time when Southern whites reasserted control over African-Americans through Jim Crow laws.
Many of the elements of Gone With the Wind: loyal slaves, the loss of the "Old South," and the horrors (for Southern whites) of Reconstruction, are fundamental to the Lost Cause, though Mitchell's depiction of slavery in particular in the book is far more nuanced than the motion picture. But as historian Gary Gallagher observes, blacks are fundamentally portrayed as
...passive and faithful to Old Massa. The implication is that the war was not about slavery and the message is also that the slaves were well-treated, happy, and did not care whether they were slaves or free people.
This contention runs counter to virtually everything historians know about the conflict. But Gone With the Wind, while exerting a tremendous influence on American memory of the Civil War, is representative of a trend that predated it by decades. It even was supported by some professional historians of the time, the so called "Dunning school" that portrayed slavery as a healthy labor system that was good for all involved. Portions of this view persist today, though modern historians, including Gallagher, have discredited almost every aspect of the Lost Cause, partly by demonstrating the historical process by which it was created.