If spammers make money by spamming products nobody could conceivably want, wouldn't you suppose they could make even more money selling stuff that someone might want?
For example, there's a big MLM industry built around household supplies -- cleansers, cosmetics, quack medicines, stuff like that. These might not be great propositions. But I bet you can imagine buying better soap or a new vitamin, while it's hard to believe anyone bites on the Nigerian spam.
Suppose you lost your job and you had to go out today and get a job in sales. Which would you choose: the sort of things spammers sell, or something like used cars or hair brushes?
Why haven't ads for marginal products driven out the spam for products only an idiot would buy?