Do defense lawyers not realize that while your average Joe Jury Member MAY be swayed by a prosecuting attorney making your client look nervous, your average Joe Jury Member DEFINITELY will think it is odd that the defendant does not even attempt to defend him/herself?
Criminal defense lawyers realize that there may be some mocats on the jury (especially if the defense lawyer didn't eliminate all the mocats in voir dire) that will put weight into the defendant not testifying, but also realize that if the defendant does testify that mocats and non-mocats on the jury will get to watch their guilty client get destroyed on the stand and undoubtedly be found guilty.
And as was alluded to earlier, that's all because prosecutors generally only take the absolutely guilty ones to trial. And not only the just the absolutely guilty ones, but the absolutely guilty ones where there's nothing weird with the case that the prosecutor can't overcome to get a conviction.
Notwithstanding all that, 12 Angry Men is a great movie.