To prove to yourself that SMTP is primarily concerned about authenticity over privacy, just try setting up your own mail server. Ensuring your mail is not blocked and/or marked as spam is an involved process of establishing multiple checkpoints re-confirming your identity. DKIM, SPF, domainkeys- each intended to establish that you sent this (unencrypted) message, and not someone pretending to be you.

Google won't hand you a packet capture from eth0 on smtp.gmail.com because you asked nicely. You would have to coerce an insider (or exploit your way in).

The only situation in which your statement applies is if you're abusing a position of trust as a network administrator. While it's true that this is possible, it's also possible for someone to break into your (postal) mailbox. You still have an expectation of privacy.