Today a small number of abusers are doing mailings with tens of thousands to millions of recipients. So we all get many unwanted mailings in our boxes each day, polluting them and making them unusable. If we take away the ability to do a mass-mailing, and require that any unsolicited sales letters be personally written for the recipient, we put a cost on Spam that isn't there today. That cost puts an inherent limit on the problem, which today exists because bulk E-mail is cheap. The truth is, E-mail marketing just isn't very effective.
Most people just discard it immediately.
Some even filter it out in advance. Imagine though if there becomes a cost on such mails, even a just penny. In reality, personally written messages cost much more than a penny, even if written in an electronic sweatshop. Good personally written messages cost dollars per message according to most business accountants. On the telephone, the ability to have a live sales conversation makes the method productive enough to afford the cost of paying the telesales people.