(And I mean, of course, the movie.) The only major, white character, that is. The blacks speak Early Southern Ebonics. Some of the minor white characters, like the Tarleton Twins, speak Southern. But Ashley Wilkes talks like an upper-class Northeasterner. So does Scarlett’s mother. (Her father talks like an Irishman, but he is one.) So does Melanie, almost. Rhett Butler talks like a city slicker from the Mid-Atlantic states. Only Scarlett has a Southern drawl, and even she does not do it very well. Was it impossible to find any actors who could do a Southern accent?!
Olivia de Havilland is and Leslie Howard was British. But so was Vivien Leigh.
It’s because the only people who knew what a real Southern accent was like lived in the South. Remember, this was before the explosion of media in the second half of the 20th century. In fact, it was only 10 years after movies acquired sound. Most of us are familiar with exotic accents only through TV and movies, so the 1939 audience was simply more ignorant on the subject. So they took such accents at face value. To the extent that there is a specific faux-southern accent that exists only in movies, but is still the most familiar one to many people. NPR did a story a while back about a highly sought-after accent coach, the go to guy who worked on all the big movies, but who’d never been outside of New York; all his accent knowledge was simply logrolled from old movies into the new ones. He had to “teach” Julia Roberts how to have a Georgia accent for Steel Magnolias. Julia Roberts is *from *Georgia.
IOW, there just was not a standard of accuracy in such things back then.
ETA: Another factor is the difference in travel between now and 1939: it was a much bigger world, and your average audience member was far less likely to be familiar with cultures, or even other U.S. cities, outside their own.
Remember the book was HUGE before the movie got rolling. The “Search for Scarlett” open casting call was an “American Idol” type big deal at the time. A really, really big deal. Everyone was on the edge of their seats wondering how the movie would be cast and who was glam enough to do Scarlett justice. I imagine a pitch perfect southern accent was way down the list of attributes they were looking for.