an answer to a recent question

Earlier this week, I was talking to someone about spam and was asked why people do it.

This article from the BBC suggests an answer to that question: 

"Our analysis shows that spam works," the team wrote.

"Among its millions of recipients are not only those who read it, but who also act upon it"

so there you have it.  The simple answer is that spammers spam because it works.  Read the article for more (surprising) detail.