Why do spammers spam?

It makes me wonder why spammers bother doing what they do. Every internet savvy user must be aware of spam and UCEs by now and take absolutely no notice of them. In fact it has got to the point where ALL users I know are even deleting bona fide mail-shots (which incidentally, US marketeers please note, is subject to different laws in the UK and EU) without reading them.

The whole exercise is self-defeating so wouldn't it better for the world if spammers would just stop? Let's face it, does anyone REALLY take penis enlargement pills seriously, buy drugs and software from some unknown source who sends you an email, or respond to pump-and-dump emails? I suppose anyone who does deserves to be fleeced and that is who the spammers are after but what proportion of the spammed recipients is that? Pretty much nil I would imagine as most if not all, internet users are pretty intelligent.