because everybody knows adc's are the only physical damage champions in the game (more specifically auto reliant, but theres still many others).
Wow your reply gave me a brain tumor.
Thornmail has one purpose and that is to counter HARD AD CARRIES, and it has no other purpose LOL. There's like, maybe one melee AD champion that would give a single shit about Thornmail and that's Fiora because almost all of her damage is reflected.
Yi? Whoa true dmg omg stack teh AS and don't give a shit
Yasuo? LOL Ult, what armor? Also has a ton of burst from abilities = NOT REFLECTED WHOA
Any bruiser in the game? Innate MR scaling, most have innate defensive abilities like shields, lifesteal, resist buffs and they all build Hexdrinkers, BorKs, mercs and hydras and don't give a shit about Thornmail or anything else really
But please continue to ask for an item that overall sucks so you can counter champions that are already struggling on the map because you can't think past your ass.