Why do people think the south is incest?

Well, I larn sumthin’ new every day!

They think we country people are uncivilized. In other words anyone who thinks what you described is arrogant.

Blame Jerry Springer.

people are ignorant and like to hear themselves talk about a bunch of nothing

There is no good answer to where the reference came from.

The concept is simply an insult that is commonly used to show that you (the insulter) is more civilized than the person that you are insulting. The idea for the insult is because of ‘backwoods’, uncouth, small communities that existed before the current levels of interstates, cities, electricity and books were as available as they are currently.

These communities did not have enough families to breed without the other person being a relative and so family trees did not have the large number of branches that we find when the community has at least 10 or more families to choose spouses from.

In some ways, current day cults, communes, and polygamous sects could employ these same backward methods of raising children by reducing the number of fathers and increasing the children.
The next generation could have many children who are all closely related, and if the community is insular, the children may intermarry.

Let’s hope that this does not happen.

I think the belief still stands because of movies and tv shows. The old moonshiners and hillbilly type people were supposed to have married or had relationships with close female relatives.
Also “trailer trash” have supposedly slept with most members of their family.
Most bad guys in many stories of the south are/were participants of incest.