But this limits promotion where actors do interviews and sell the movie to the public. It also limits an actor doing something crazy that tanks a movie like a tweet.
But for plenty of other masters - think Cassavetes, Mike Leigh, even PTA - the actor's outstanding talent and instincts bring something to the script and vision that is outside of their prescriptive powers. Their focus is essentially setting up a framework for magic to happen inside of.
As a choreographer myself, that's not necessarily a problem but rather a feature: it depends on how the director creates. Often you want what's unique to the performer, you don't want them to do something that's exactly like what you envisioned but whatever their interpretation/vision of it is, the "imperfectness" is what makes it interesting and rich.
Also, some great lines in movies came from actors ad libs. I hope there will always be some space for mild hallucination; without improvisation we wouldn't even have jazz.