I agree but the problem is that saying 'we need an unknown actress' is basically equivalent to saying, 'there are no women in Hollywood that we know of who would be right for this role so hopefully there's someone who we don't know of.'
Well, there's also the idea that an actor's personality and popularity and surrounding buzz would take attention away from the movie. Remember a few years back when Tom Cruise was jumping up and down on Oprah's couch and shit? It didn't change his abilities as an actor, but there was a lot of buzz surrounding him that studios didn't want to touch at the time. And that even transferred to Katie Holmes. They say that there's no such thing as bad publicity, but when all the buzz is about whether or not your star is insane instead of being about your movie or even his/her role...well.
And there's the idea that the movie might be viewed as a "star vehicle" for a particular actor, based on the roles he/she is best known for. Playing the role might be well within the performer's abilities; even playing it well. But perhaps the audience has a particular expectation of a performer. Like, if you cast Jason Statham in a movie, people are gonna expect him to fall out of a plane and start karate-fighting people the moment he lands...even if the movie is supposed to be a true story based on a bank heist which has practically zero karate-fighting in it.
The performer might be the best one for the role, but maybe the perception surrounding him or her governs public expectations too much.