Because it's profitable

They spam because the costs are so ridiculously low that it almost can't fail to make money. If it didn't, they wouldn't bother. But that would require the tiny number of idiots who respond to spam to stop buying crap from spammers.

Now, if ISPs could somehow detect when a user clicks through from a spam to a spam site and buys something, and cut their service off immediately with no refund or recourse, people might learn that buying from spammers is Bad and Hurts The Internet. Maybe name and shame the buggers too, just for added effect.