I think stray magnetic fields were more of a problem back then. Fridge magnets, magnetic clasps on wallets and bags, magnetic clasps on notebooks and binders, etc. There were lots of hidden opportunities for corruption.
These days, nobody worries about the mag stripe on their credit card getting wiped. But it was an active concern back then.
I'm more annoyed when a store doesn't take Apple Pay. The clerks always seem embarrassed when I ask because I'm sure I'm the 900,000th person to inquire. I find it so odd that the Krogers in two cities I've been to recently don't do contactless payments at all.
I do remember cassettes had different quality magnetic strips but I don’t know if the same was ever true for disks too… so it’s probably bonkers. I’m just throwing some ideas around without much thought. :)
It might just be a case that college kid me was less careful with his disks than you were.
More recently though, I have unboxed my old collection of 3” (not 3.5” but literally 3 inch) floppies for the Amstrad CPC 464 and they almost all worked despite being nearly 40 years old. So I definitely believe you when you post about your success rate.
Maybe consumer floppies were of a lower quality than published ones? I don’t know if that’s true but I do remember having a really low success rate with consumer floppies (it got so bad I’d often write two floppies just in case).