Well, why are good actors so sought-after? What makes someone an actor whose likeness people seek and what makes a B-tier actor?
Part of it is inertia and random celebrity status, having an attractive or interesting face etc., but part of it is also the raw knowledge of when to apply certain microexpressions, how to gesture etc. i.e. how to do the acting itself. To be a convincing, charismatic etc. actor it's not enough to wear a digital mask of a celebrity, the underlying actor still needs to act well. That may not be so important for certain types of shallow movies, but it certainly is for deeper drama films etc.
It's similar to today's text generation where you may be able to generate sports game reports, user's manuals or travel brochures etc. but not really those where you need high level decisions, like applying the appropriate expressions to a real-world event, taking into account all the context, like writing a poem about your feelings reflecting on some recent real-world event.
I'm not saying humans have a magical power that can't be implemented in silicon.
What I'm saying is that deepfakes as they are today are not sufficient to replace actors. You'd need a higher level puppeteering AI that would take the whole storyline and script into account to come up with the right ways to express the appropriate emotions at that moment in the film and could take the director's instructions regarding his vision of how the drama should unfold etc.