So I may have learned something to-day. Archives of email lists and newsgroups are I am sure much more attractive targets for spammers than keyservers. Those involved with servers associated with email or newsgroups have fairly easy access to tons of addresses every single day. I am not aware of any documented report about keyservers being huge sources of spam.
Web sites collect all kinds of information on visitors, including tracking what one looks at.
Spam is only really effective if the spammer knows something about you. The little spam I get, is targeted most of the time. It comes from visiting Web sites as far as I can see. For instance I visited Web sites lately to look at home gyms, and in the next while received some spam about exercise equipment. A list of email addresses from a keyserver tells the spammer absolutely nothing about the interests of people.