Well, that's not true. A person can speak all they want and not everyone even decides to use an attorney, especially for misdemeanors.
But I am firmly in the camp of "I am going to speak if I am innocent," even if my attorney says otherwise. I make a terrible client for an attorney, btw. And I've seen attorneys do the same thing I just said I'd do. And judges. And Sheriffs and Sheriff Deputies and other LEO's.
IOW, many of us who are more familiar with the system DO go ahead and talk. Indeed, we also people later found guilty out there announcing their innocence (with and without their attorney's oversight). I'd fire any attorney who wouldn't let me do it and actually have an attorney who agrees with me (I've not needed him for a criminal case, thankfully). But I know someone who did use an attorney who had her stand in front of mic's and proclaim her innocence and explain what happened.
You stated this as if it's universal in the US or part of the law that the defendant "doesn't speak. Yes, they do sometimes, they can, they are not prohibited, and not all attorneys tell their clients to remain silent on every point.
IMO. IME. I am writing for the benefit of our non-US citizens who may take what you said as being part of the legal system, when it is in fact an opinion. I am also stating my opinion. If you can find a law that says a criminal defendant no longer has freedom of speech, that would be helpful. Is there a state that has tried it? I am unaware.
IMO.