...Why?

I'm not sure but I've heard them being described in that way. Maybe because they are literally cold blooded and people fear snakes, so they have the cultural association of sociopathy attached to them.

I wasn't saying it was a good description, I was just saying it is a description people would use. People kind of anthropomorphize everything, children I've observed pretty much do assume on some level animals have human like minds but that they chose to behave differently from social norms.