The answer is simpler, ads are the support model for free services. If you want adfree, you need to pay for it. The demand curve for adfree however seems to be

The answer is simpler, ads are the support model for free services. If you want adfree, you need to pay for it. The demand curve for adfree however seems to be almost flat. This means spammers can survive on incredibly thin margins. Spammer unit economics don't make sense for message costs greater than 0, even very tiny costs. Until people are willing to pay more than $0 for email, they'll always have spam.