As I posited elsewhere in the thread (source: I'm old), a big part of that isn't even reduction in quality of materials but rather change in type of materials as new textiles have been invented.

I can tell you, my socks and drawers may not last as long as they used to, but god damn it, they're the most comfortable sets of them I've had in my entire life.

You don't want to know how it was wearing boxers, briefs, and socks in the 80's/90's, because it was bad and uncomfortable. I remember being embarrassed because I felt like I was endlessly adjusting my dick in them due to discomfort.

Modern socks and underwear are made from much lighter and more comfortable material, which in turn means that they simply don't last as long because the material just isn't as sturdy.

I can tell you when I'm underwear shopping I aim for comfort over longevity of material, because I prioritize basic comfort over the underwear lasting forever. I'm sure I'm not the only person who approaches it this way, I'd wager the majority of folks prioritize comfort of undergarments over longevity.