You could argue that telepresence robots work and don’t need replacing. That may be true, however, a disembodied head on a stick is not the most engaging of things to watch on a stage.
A giant projected talking head isn’t much better. When Stefan is on stage, dressed as Steve, he can move around the stage gesturing and gesticulating just like a real human. It’s almost as good as having the speaker there on stage. In some cases, thanks to the novelty factor, it might even be better than the real thing!