I think the appeal of spokespeople is that they're actual human beings. Knowing they're computer programs would only appeal to the Hatsune Miku types. The only way they could get away with marketing an AI spokesperson like a real one is by hiding the fact that they're an AI, which in itself would lead to a huge controversy the moment the cat's out of the bag. As it stands right now, an AI spokesperson is no different from a mascot.