Because the user is hitting the opponent. Armor is for defense from enemy attacks. Thorns aren't an attack.

I'd agree with this logic if we would talk about Dodge. Dodge does actively work against an attack and you can't possiby dodge thorns. But Armor supposed to work against damage. Any damage. At least any physical damage. This is what decades of gaming has taught me. And this is what common sense dictates. So I was very surprised when despite me stacking armor I was still getting full thorn dmg.

On one hand, I agree it'd make sense if armor stopped thorns, on the other hand there are already multiple bosses that hard counter thorn builds and I don't know if they really need a nerf.

On the other other hand it would be nice to have a way to counter opponents thorns if you're going for a spam build instead of just being screwed if you randomly run into an opponent with lots of thorns.