Out of the several million, probably more, spam emails a day, they are probably going to find at least one or two people who are interested in the product. As it costs nothing to send email (other than the cost of having a computer switched on), the return is worth the investment.
You can do your bit by NEVER replying to a spam mail, not even to abuse them. It only confirms that yours is a valid e-mail address and you might find an ever increasing amount of rubbish filling your inbox daily. These low-life will not hesitate to sell valid e-mail addresses to others. Inform your ISP if you get spam regularly from one recognised source.