It's really not. Squishies are squishy. Everyone kills them. Why shouldn't they? I mean...REALLY. Are you serious?
ADCs do not "counter" everything. The job of the attack damage carry is to deal damage. Damage kills things. All the things. It's the defining trait of anyone called a "carry" in this game. They are MEANT to be the backbone of their team's damage-dealing capability, IF they can survive long enough to do so. The other defining trait is that they sacrifice nearly all defensive capability or itemization in the name of doing damage. They kill everyone, but they don't outright delete anyone, and they are just as vulnerable to being killed, because they are also squishy. The question is, how fast do they kill?
The reason for that question is assassins. It is their job to do enough damage to kill a single target as quickly as possible — more quickly than an ADC, and the enemy ADC is usually their primary target. An ADC can't 1v1 an assassin, because their whole thing is "I'm going to kill you in particular, right the hell now". And then they have cooldowns, because they're only supposed to delete one target at a time, not your whole team. (ahem Kat, what do you think you're doing? That's wrong. No. Stop.) But see, while assassins mainly kill squishies, that's really most of the team, anyway. ADC? Check. Enemy assassin? Check. Utility support? Check. Burst mage? Check, if they don't have Zhonya's ready. Even semi-tanky bruisers, if they're feeling gutsy. Assassins kill anyone who's NOT a tank, and they do it much faster than an ADC.
The job of a tank is to be bulky enough that nobody can kill them quickly. They survive the assassin's burst with plenty of room to spare, and then maul their face while their cooldowns are ticking, because everyone kills squishies. The thing that makes ADCs able to kill tanks is the lack of cooldowns. It takes them longer to match the damage of the assassin's burst, but then they don't stop.
Going back to your earlier question, it's bad for a tank to do 300 DPS to an ADC even through their lifesteal JUST BY STANDING THERE, and more if they actually attack, because ADCs are squishy (you know, that trait that means everyone kills them if given a chance), and they can't do their ONLY JOB if doing it gets them killed without their opponents even doing anything. This REALLY shouldn't be that controversial as a concept.
It's not so much that ADCs counter tanks. It's that tankiness counters every kind of damage that's NOT sustained DPS. It's that ADCs are the only ones tanks aren't supposed to counter (except by surviving a bit longer and being able to apply CC to give an assassin an opening in a teamfight; NOT by killing them alone, because only an assassin or burst mage should do that). And then Thornmail comes into the picture, and suddenly the only one who could beat them alone now kills themselves trying. Thornmail is overtuned. It deserves the nerf.