My guess is that there must be a return on their effort - in other words enough people must click-through to make spam worthwhile.
On one level they are a nuisance, but if there are enough spam emails, they are clogging your Internet connection. So, are there enough?
Here is real data from a Spam Filter here in Australia - you decide.
Total Emails received = 9.5 million
"Genuine" Emails = 2 million
Spam Emails = 7.5 million
This means more than 80% of the email arriving at this domain is spam.
Having an absolute fix for spam is difficult, as it involves technical and legal remedies. That said, I notice that Spam Filters are now doing a great job. For example, I have a Hotmail account and the Spam arriving in my inbox is zero. If more and more spam is blocked, then hopefully, the sources will diminish.