I'm both surprised and confused that Annie is not known for having rotating child cast like many other musicals (eg Matilda & Billy Elliot).
It's a lot for children to be at the theatre 6 days a week!
Anyone know why this is so uncommon with Annie?
Depends on wich country you are in. If the country has strict child laubour rules, like for example Great Britain or Australia, they always have rotating child casts. In Matilda it was also decided from the creators of the show that they did´nt wanted one child to be THE Matilda, since they would have to replace them every year, or six months. So they decided to go with four of them, and a quite secret schedule:-).
Honestly, when "Annie" first opened in 1977, it was not a common practice to have alternate casts. This is a fairly new thing. Children in theatre were always treated like adults and professionals, so no one ever questioned or doubted that we could handle doing 8 shows a week - it's what every Broadway performer did, so we worked just as hard, if not harder (considering we also had to attend school during the day).