There has never, ever been any expectation set that internet email is private. There have been many examples in the broader media that show how one might compromise email. Also, you have no way to assess the quality of the email service provider, network provider, or client environment.
Postal mail is more secure for 99% of the public for several reasons, including:
- A paper envelope is tamper-evident. My dad used to correspond with radio operators in the Warsaw Pact... envelope tampering was trivial for me to detect as a 5-year old.
- Stolen mail is stolen. You don't get the message. Detecting a pattern of missing mail is pretty easy.
- If you're not a police organization, tracking postal mail metadata is risky. Bystanders will notice somebody rifling through a mailbox every day. There really isn't a way to surveil outbound letters.
- It's a serious felony to tamper with mail. Linking physical mail theft to a perpetrator is pretty straightforward. Also, Postal Inspectors take mail integrity very seriously, sometimes too seriously. With electronic crimes, you probably have a 1/100 chance of finding a cop who understands your complaint AND has the means to do anything about it.
- It's much easier to implement physical security practices/procedures that keep secrets transmitted by mail secret than via digital means.