It is already difficult to be a tank against physical damage as it is nowadays, due to so many AD champions having ways to bypass armor, be it armor shred or penetration, be it true damage, or be it even magic damage.
If anything, more AD champions should be like Riven: Not having any innate way to bypass armor, instead having to look into items if they want to actually bypass armor stacking.
Not to mention the armor penetration and reduction items are very strong, and we have a ton of them. Divine Sunderer, Black Cleaver, Eclipse, Lord Dominik’s Regards, Mortal Reminder, and Guinsoo’s Rageblade. Compare it to the AP champions, which only have Void Staff for the most part. Very few AP champions have synergy with either Riftwalker or Guinsoo (which would also allow them to bypass MR). Lord Dominik is one of the worst to face because it also punishes you for stacking HP (especially when combined with BotRK), which would be the counterplay for tanks against armor shred.
I once got shredded as a Malphite by an Akshan. When I looked into the damage done, he did almost half magic damage to me. And no, he wasn’t building full AP. In fact, that was BEFORE they added AP ratios to his passive. Akshan is just one of those champions that is difficult to build against because he has a lot of mixed damage.
In fact, full AD teams are not an automatic loss against teams with a tank like full AP teams are. Full AD teams are supposed to have a very hard time unless the opponent has no tanks to take advantage of your team having poorly balanced types of damage. But thanks to the multiple ways to bypass armor we have nowadays, this is not the case.
Yeah, there is just way too much pen and built in %hp damage these days. Combine that with built in armor shred and you can have virtually no armor vs, say, a kogmaw with guinsoos, Bork, and LDR if he hits q on you. Kog is an extreme case i suppose with the %hp damages and having shred on q, but the point stands. For instance, shaco with eclipse, bork, LDR, Prowler’s and Cleaver(which is all a good build except for cleaver, which is just ok), basically takes all your armor away. 20% armor pen on eclipse,+30% from ldr, then 30% of the remainder being shredded by cleaver, combined with like, 36 lethality. I’m not saying i can assassinate tanks, but if one is pushing by themselves, i can easily put them to half health with shaco. I can assassinate them from about half hp, doing an absurd amount of damage just in the hail of blades window.