I agree, but the problem with spam is that it is just so goddamn cheap to send.
That is part of it...
It's not an economic problem like drugs are
I will argue that at the root they actually are the same. A spammer and a drug dealer have in common the motivation to make money. A spammer cares no more - or less - about the condition of the customer than does a drug dealer. For that matter, plenty of spammers effectively are drug dealers, spamvertising for sites that sell (often counterfeit) drugs online.
because it doesn't require the massive resources a successful drug empire does
While spam does not require much for resources, it does require an economic motivator. Spammers very rarely are webmasters themselves, they usually are paid by other companies to send out spam.
it's almost impossible to keep these guys down.
That's not entirely true. As I mentioned before, spammers do what they do for money. If they don't get paid, there is no incentive for them to send out spam. Disrupt the money enough and the spam rapidly drops. When we instead keep trying to come up with alternative hardware and/or algorithms for spam filtering and detection, we just encourage the spammers to find new ways around it so they can get paid.