As a University of Georgia graduate, I think one word summarizes why the girls were dressed up for the football game you attended...tradition.
Southern girls want to look "classy" at the football games sometimes because they have been tailgating with a prespective boyfriend's family and want to make a good impression and other times because what better place is there to meet guys than a college football game:-)
I found a great quote on the internet that sums up football in the South:
"In the East, college football is a cultural exercise. On the West Coast, it is a tourist attraction. In the Midwest, it is cannibalism. But in the South, college football is a religion, and every Saturday is a holy day." – Marino Casem, Alcorn State University
That basically says it all. Some people dress up for church. Southern girls dress up for football.