And no service offering encryption has existed since 1755? Because that is required for your argument. Otherwise you simply send unimportant stuff via USPS and sensitive/secret/important stuff via non-USPS.
My vote, my taxes, my REAL-ID driver license, passport, credit cards, phone SIM, checks, 401k statements, etc. have very recently been sent via USPS. Do you consider this unimportant stuff?