Mailchimp charges $12K/year for 150K contacts and 180 emails/contact/year.
(Above that is "call us" pricing.)
At that rate, it is about 10c per contact per year. Substack has 500K paying reader-subscribers, 10-20x that in "total readership", whatever that means, and "millions" of readers (but I believe those are web viewers).
Managing the reliable sending of millions of subscriber emails is expensive.