It's a classic question with a logical answer. Read
Cain's Wife if you're not sure about the matter. Let me know if you think I have it wrong.
This is a question I have asked so many times to many preachers with no answers. In fact, I got kicked out of children's church when I was 13 for asking this question.
"Preacher" is a pretty broad category, and not all of them are so smart. So, did you go read my answer?