I’d have to go along sethdallob’s ignorance hypothesis. Whenever I get calls to clean up viruses, they’re usually one of two types of calls:
“Ummm, I did something and now my computer is acting all strangely.” Did you open any unusual files? “Maybe. Yeah, I might have.”
“I think I opened a virus.”
The former type of call usually comes from people who aren’t particularly tech-savvy and don’t really understand what a computer virus is, how it works, or how it spreads. Hence, they don’t understand why an annoying warning message dialog box has opened informing them that they should not open a message containing a funny joke or an interesting love letter.
The latter type of call usually comes from people who do understand what a virus is, but, for some reason don’t connect the warning message with “don’t open this virus file.” Sometimes they’re in a hurry to scan through their email and aren’t really paying attention. Other times, they’re curious about what the file is (“oh, it’s not really a virus, I don’t think. My best friend wouldn’t send me a virus”). And so on.
Add to this the fact that antivirus programs don’t always catch all viruses immediately and you end up with a decent chance for a computer virus to spread.