I was talking to a used book dealer once who said next to National Geographic, old Playboys were the most worthless of all the magazines they had. I've found that out too, when I sell things every couple years on E-bay they're the only things I can't get rid of even with the centerfolds intact. It might be that because before 1975, it was pretty much the only thing resembling pornography anyone had access to, so millions of copies still remain in peoples' homes.
Nevertheless, abundant as they are, 1975 was still more than 40 years ago, before at least half the people I know were even born, and the material is still obscure to most people.
Virgil Partch from the first issue in December, 1953