Did you mean that it's not common in root beer? Wiki says that root beer has little to no caffeine.

Root beer is so sweet that I would think it could mask the bitterness. I wonder if the plants that originally were brewed into it just didn't have any caffeine.

Do the ingredients to coke naturally have caffeine, or was that a substitute for the original alkaloid? I.e., is decaf coke because the removed the caffeine, or is caffeinated coke that way because it was added?