It's not so much that they are stupid as some people (many?) get a computer just to use e-mail and do basic word processing and web browsing.
Especially older people...they tend to really think the spam e-mails sent to them are...specifically for them.
Couple days ago my friend called and told me you won't believe how stupid Patty's father is. Patty is his cousin's wife.
He tells me that he got one of those e-mail from Nigeria where you need to just send a ceratin amount and they will send you back a lot more. Like someone is in prison or some shit...
So...(and this isn't a joke) the guy sends them his life savings of $30k. He gets a cashiers check and goes to the bank to cash it. The FBI contacts him couple days later asking why he is doing this and tells him it's a scam- not to be involved in any way. They also told him he lost all his money and there was nothing they could do.
Like the next week he got another Cashier's check for $120,000k - this time the bank is Canadian.
He calls the Canadian bank to ask if it's a valid check. They tell him it is.
He goes to Canada to cash it. FBI was waiting there. Now they are thinking he is in on it. He was desperate. Now he is in a Canadian jail and his savings is still gone of course.
Fucked up, huh?
Older people who are just getting a computer are most vulnerable to those very official looking e-mails made to look like they are from banks, paypal, ebay, etc...
Ignorance is bad when you're hooked to the 'net.