Thornmail's common counters (as I believe was a 2.1k item) were

All of which cost significantly more up to a 1,6k gold difference, Not many ADCs even build Bloodthirster or Mercurial until their 4th or 5th item, since they need something like IE > PD > LW to begin to even do damage to the tanks, and now they have to build a huge lifesteal item too or the tanks will start punishing them for doing damage. On top of that the Magic Resist from Mercurial Scimitar is very minimal and Maw isn't really built on ADCs anymore.
It wasn't impossible for Thornmail to do 900 damage, and actually I remember specifically a week where nearly every game I played ADC has a Thornmail, I would get fed and do really well, and then get shutdown wondering how I died to their 1-7 Jax. Did I play the duel out wrong? Did he hit a big powerspike? No, Thornmail just did anywhere from 500 - 900 damage and I had to immediately rush Bloodthirster and BoRK or just not even auto attack their Jax. I should of checked his items to be fair, but it wouldn't of changed the outcome of the duel. While Jax isn't the best example as he's a Fighter and clearly should be able to outduel a lot of ADCs, I've had a lot of cases like this with Sion. How is this huge meat shield doing damage, I thought this was post-rework Sion? Oh, Thornmail. It's not even in 20 so auto attacks, the item returned 30% of the damage you dealt to them which is insane against a squsishy little ADC who can't even risk to buy defensive items or they may not even do damage.
The item may not seem like It was that strong, but on a Bruiser it was stupid. Either you rushed Lifesteal or you just didn't fight them. It punished you for doing well and only got worse as you did well. Now on specific Tanks like Sion who rush full Tank items, It's a slightly different story but even if Thornmail only did 400 - 500 damage, that's around 1/4 of the enemy carry's health when they're at they're max base health. not including your own damage on top of that.
Since ADCs typically only have around 1.9k - 2.2k base health, and Thornmail could easily be bought as a second or third item, while ADCs can only afford to buy Lifesteal or Magic Resist as their 4th - 6th item (none of this including Boots), It's no surprise that Thornmail did more damage than what you think. It was either you built a significantly more expensive item and risk doing no damage at all to begin with, or you get blown up by an item.