I don't think they are intelligent. An intelligent person will never live a life of a scammer. He will find a way to bring bread and butter in a legitimate way so that the business can sustain for longer term and make a positive impact for him and his family.
I believe scammers are lazy bastards. They can't do a job or work to earn a living but instead, they think of scamming others and eating what they their money that they have earned with hard work and dedication. They might not be intelligent, but they practice a lot which makes them good enough to pull off the scams on vulnerable people, make them face an experienced person who has quick thinking abilities and they would get shit for their tricks that some people consider intelligence.
They target vulnerable people because they know they aren't going to get anything if they try to scam people with knowledge, so they always look for weaknesses among their preys and then trick them with their words and actions which can be obvious for someone smart but aren't for someone who doesn't know much about these things which is the reason why they get success.