Hitchhiking in the us is dangerous for drivers as well as the hitchhiker. So maybe it just carries over. Not excusing rudeness.
What conversational situation were you in to be conversing with foreigners about the economy?

I think it boils down to people not trusting who or what they are not familiar with.
I'm a White South African and I do not stop for hitch hikers regardless of their colour unless I know them personally.
There have been various cases in S.A. where criminals get women to stand looking helpless by the side of the road. When a car stops then the criminals appear from hiding places and...well you can imagine what happens then. Sometimes the women are part of a criminal gang and sometimes they are forced to do it at gunpoint.

So I think the problem is simply distrust. The world is becoming more and more about protecting one's own family.

I would say that represents a common American view, not sure I'd extend it to Europeans quite as much.

I wouldn't hitch hike in the UK these days so it must be safer there

Can ya blame em with all the kidnappings

why do you hitch if you have a car ?

In general Americans have become wary of people trying to scam them. Tourist predation is not really a rare phenomenon around the word. Pair that with racism native to the US and the fact that actual exposure to Africa prior to travel largely comes from fake Nigerian bankers and lawyers emailing us, it's easy for some to expect the worst.

Well africa in some areas is known to be dangerous for white women to dwell in.

I imagine it's dangerous for tourists in general because they surely have money on them.