I kinda have the oblivion NPC attitude.
"I don't know you and I don't care to know you" (so I just avoid people in general and don't view people outside "fellow human", doesn't matter who or what they are, I don't want to interfere with their life or them with mine).
But if they do interact with me then it's "Hello" (ie be Friendly, but try to exit that situation promptly).
So there is never a case where I will be interacting with a hitch hiker, so I'd never pick one up.
I do think it is just the perceive there are more crazy people free in the world. Some good person would probably pick them up, but that's not me. I also don't think I've ever seen a hitch hiker in my life.. So hard to do it if they don't exist.
Society also pretty much told us not to, with stranger danger and all that stuff. A lot of things that use to be acceptable or even expected, are frowned upon now.