It's really not. At the very least, by allowing an unfamiliar person into the car I've made the trip less safe for both of us, because now my attention is split between them and the road.
Am I saying dangerous crack-addicts don't exist? Sure they do, but the idea that you are more likely to be hurt picking up a hitch-hiker than in a convenience store, or by another driver is completely without evidence.
Do you have any reference to support this? Or is it just personal fantasy? If you look at the fbi.gov article mentioned (I linked it in a lower comment as well), you may notice that the hitchhikers were most often the victims: The victims in these cases are primarily women who are living high-risk, transient lifestyles
That you nevertheless think this statement to be true - I think rather supports my point about it being a question of culturally conditioned fear and stereotypes.
Anyway, whether hitchhikers are more often victims has no bearing on the question of whether picking up a hitchhiker is more likely to be dangerous than encountering some random person at a convenience store. Most people are quite certain they aren't going to murder a hitchhiker, so they don't worry about that in deciding whether they'll pick one up.