It's a very good point. But the only problem with running mass mail from such servers is that most hosting companies now have limits in place. Also, if the database is not clean it can result in a lot of email bouncing back. Another problem is spam - it's way to easy for consumers to click spam which can result in the mail server being blacklisted. Which would result in wasted email marketing campaigns.
I have done many PHP mass mailing scripts but over the last 3 years, I have decided to stop them as I spent way too much time fighting to keep the mail servers from ending up on the black lists. Also, never worked with spam email lists - but still experienced problems.
I am not sure how Mail Chimp and other similar companies get around such problems but they do a better job then I could. So, now I focus my time on integrating these systems, creating funnels and actions.