This would be true if every car driving by a hitchhiker was equally likely to pick him/her up, but that obviously isn't the case. The only cars that stop are driven by people who have a reason to do so.

While I agree that it is unlikely you will get murdered by someone picking you up (much more likely that a bored, lonely trucker will offer a ride), it is absolutely more dangerous than an average interaction with a stranger. It is foolish to say otherwise...