"Nigerian scams" are so well-known that we call them, well, Nigerian scams. So why don't the scammers update their tactics a bit, rather than coming right out and admitting they're from Nigeria right there in the body of the email? The answer, according to a Microsoft researcher, is simple: The scammers want to target only the truly gullible. By making their emails sound completely unbelievable (you'll make millions overnight, we just need a tiny bit of money from you…), these scammers ensure that anyone who actually responds will be their ideal target.
Most of us have been around the Internet long enough to recognize one of these scams (real name: "advance fee fraud") from a mile away. But if a scammer is able to ensnare someone gullible enough not to catch on, it's more likely that the entire scam will work on that person, and the scammer will make money. Sending out the emails is easy, but going through with the actual scam is much harder—and more expensive—for the scammers, so they don't want to waste their time. Gizmodo and Computer World have more on the researcher's findings.