I suspect that if potential jurors were punished for revealing their prejudices, the defense would have major grounds for appeal after conviction: "We weren't permitted to identify the jurors who were already against us!" And the voir dire is part of jury selection before the trial; it's intended to identify any basis for impartiality. You will be excused if you know the defendant, the witnesses or any of the attorneys, and you will almost certainly be excused if you have been a party to a similar case or a victim of a similar crime, or if you are a lawyer or a police officer or public official (any of whom might possibly sway other jurors). Somebody who says "I can't be impartial" would be more trouble than they're worth for all sides.