Fax machine -> fax machine : secure And fax machines are easy to use. Securing document exchange by email is a PITA for 99.9% users. Explain to your mother how to use PGP/GPG to encrypt the email she have to send to financial institutions, government, ... And imagine the government employee receiving the encrypted email. He will probably just delete it. :)
Well, completely unencrypted, but depends on what you consider the threat to be.
HOWEVER doing that is a hardcore crime. You can be sent to federal prison for years for doing that. So the USPS is not secure in any sort of mathematical sense, but in practice most people trust it most of the time, because security isn't just about technical implementation details like encryption.
Similarly wiretapping telephone lines may be straight forward, but you'd have to be exceptionally daring or stupid to actually do it.