I mean, why don't we have GAI?
I don't know how to describe it except that performing a role in a fully believable way is akin to writing an engaging believable novel, which AI can't do, and is nowhere near doing, either.
If you try taking a beginner's acting class at an acting studio in e.g. LA or NY, you'll quickly see how difficult and artistic good screen acting is. How hard it is to perform lines in a way that seems like real life, as opposed to seeming like bad acting (melodramatic exclaiming of lines).
Of course, a lot of people don't understand just how difficult and skilled screen acting is, because the entire point is that you're not supposed to notice acting at all. You're supposed to be fooled into thinking these are somehow real characters.