I once witnessed a fellow fall for a lottery scam, all but certain that his investment would pay off big.
Scambaiter activists attest that it's not easy saving the most gullible. In one case the victim (at the urging of the Nigerian scammer) became convinced the good Samaritan wanted the $50 million transfer for himself.
Recently there was a report of a investigational medical drug that had the side effect of increasing gullibility and risk taking. There's probably a genetic or chemical basis.
It's not simply ignorance. I've seen some people fall for the same scams over and over.