Second, a postdoc is still a training position. Medical residents are also not paid much better than mail carriers either.
Third, postdocs select for two types of people: those who want R1 professorships and those who need a position to stay in the country. Both of those groups drive down wages. I.e., it's sort of meaningless to compare a group of people who are working for the money to a group of people who are working for a shot at a seat in the upper tier of a tournament-style profession.
Fourth, both annual and lifetime earnings of PhD holders (at least in STEM) continue to outperform high school graduates (including mail carriers).
There's an oversupply of people interested in R1 faculty positions, but that doesn't mean there's a oversupply of phds per se. The labor market seems to absorb these people quite readily and at apparently high salaries as soon as they decide to quit academia.