There’s a lot of things wrong with the way email advertisement works.
Chances are you’ve received an email advertisement sometime. Not necessarily the real spammy ones, but legitimate-looking emails from actual companies. How they got your email, you have no clue, but they’re sending you their junk anyway. Good thing those official “newsletters” come with unsubscribe links, right? Well yeah, sort of. They might send you like ten follow-up mails to confirm your email’s removal from the list, and it make take weeks to actually happen, but technically they’re obliged to remove your address from their list. But… they still have your email!
As far as I’m aware, there are no rules specifying what happens to your email address after a company removes it from a mailing list. That has the fun implication of allowing companies to pass your address on to someone else after they’re “done with it”. Or, even smoother, organizations that provide email services could just shove your address into another one of the lists they’re publishing to.
Which is why sometimes there just seems no end to all that junk mail, even though you consistently unsubscribe from it.