Why does Scout and Jem call Atticus by his name instead of dad?

Scout and Jem are more formal with their father and find it more appropriate to call him by his first name. Scout mentions that she has a courteous father but he is sometimes distant.

So it and Jem call Atticus by his name, instead of dad, because it is a sign of respect, they refer to be more formal with their father and it shows that Atticus and his children view each other as equal to each other.