Who delivers those letters? In the U.S., and many other countries, it's the government.
You're literally handing your letters to the government and asking them to deliver them to the intended recipient without peeking. In short, you are trusting them to do the right thing, because they certainly could look inside the envelope if the really wanted to.
Legal safeguards are your only defense against the government here, but we the people generally consider it a solid safeguard.
But if it were to even be suggested to send email using government-provided networks (even encrypted email) you'd be laughed right out of any hacker con you attend.
It's a pretty surreal difference IMHO.