Which is part of the problem.
But I would suggest strongly that coordination IS the key to killing a Tank, the idea that ONE Champion can just destroy a Tank is wrong to begin with.
When you get an ADC dealing as much with a Single Basic Attack as a Mage is getting for his Ability that has a Cool Down... it gets sort of idiotic.
What SHOULD kill a Tank is a mix of Burst and Sustained Damages of varying Types. I mean, that IS the reason of being a Tank... to draw the Focus of the Enemy Team. Not to draw the focus of the ADC who deletes you then moves on to everything else.
We also tend to think that Burst Damage doesn't work... but it's a large part of it. If Katarina can Ult after the Tank has used his Disrupt, she can take a HUGE chunk out of the Tank. Sure, she likely won't kill it unless extremely fed, but she'll take the Tank down to a manageable amount of HP that others can finish off.
The only reason a Tank should ever die 1v1 when on equal footing is when they find themselves caught in a Kite, to far to engage but not far enough to disengage, slowly being taken down. This should take a large amount of time, as the Armor vs. the AD should be High and the HP Pool should be Large. So even with this the ADC should have to decide if he just disengages to safety or continues after the Tank in this dance while he COULD be getting flanked.
What it SHOULD NOT BE is the ADC grabbing a Last Whisper Item and instantly chunking off 45% of the Armor built (plus Grievous Wounds to negate 40% of all types of Healing, if going Mortal Reminder), and then being able to waltz around and 1v1 anyone in short order.
Think about that, 1 Item an ADC buys for 2,700 Gold instantly invalidates 45% of the Gold spent on Armor and 40% of the Gold spent on Sustain. 2,700 Gold does that, instantly.
And THAT is why the ADC beats the Tank, because with 2,700 Gold an ADC can turn 15,000 Gold spent on Armor and Regen into about 7,500 Gold. Can INSTANTLY turn a 6 Item Full Tank into a 3 Item Tank.
Obviously, a 6 Item ADC should crap all over a 3 Item Tank.
This is where the entire 'ADC beats Tank' comes from... the fact that even before Lord's and Mortal, one Item could ignore 35% of the Gold/Items a Tank Built. Even then it was like a 6 Item ADC vs. a 4 Item Tank.
The combination of needing to Stack Armor for it to truly be effective, and the Diminishing Returns that Armor brings... just to have 1/2 of it negated by 2,700 Gold, is stupid.
Concentrated Damage, i.e. FOCUSED Damage, SHOULD be what kills Tanks, both of the Burst and Sustained variety. The Trade Off is that the Tank itself shouldn't be effective at killing anything (it's why we have Bruisers, Brawlers and Juggernauts... to deal Damage... the TRUE Tank is just there to disrupt and absorb focus).
Which leads to the common 'ignore the Tank, focus the Carries' mantra. Because, in theory, the Tank shouldn't be able to kill you unless you let him on top of you and forget he's there... so you take out the threats that CAN burst you away into nothingness... THEN use the remaining TEAM to either Finish the Tank, or ignore him as you push an Objective.
Instead now it's Pop the Tank, then GO! Because 2,700 Gold renders a Tank completely useless against some one that he specifically BUILT to be able to withstand, thus wasting every bit of Gold he has spent the entire game.
It's down right idiotic.