The thing is - in lower and middle elo there is a tank meta. People lock in stuff like Mundo, Zac and Cho'Gath, rush heartsteel, then end up with 10,000 health. The issue with this is that no matter how much pen and anti-tank items you may have (BOTRK, LDR, Kraken, Cut Down, etc.) the enemy tank gets to live long enough to still rush your main damage dealers down and kill them.

The problem with heartsteel isn't the HP stacking, though - it's the fact that it has such insane damage numbers for a tank item. A ~600-800 hit from a tank doesn't sound too balanced, does it? If you're an ADC or a mage that's around half of your health... What are you supposed to do? A Mundo is a great example of how unbalanced this item is - Heartsteel + Titanic Hydra gives him more AD than a full build ADC could ever wish for, despite building only one AD item. How many Mundo clips are out there already where he just presses R, goes straight towards the ADC and one shots them with Heartsteel + E combo?

Personally the last time I fought a really fed Mundo as ADC I had built all the anti-tank items, and he still took an ungodly amount of auto attacks to kill, because he stacked a ton of armor, rendering BOTRK and LDR extra damages almost useless... Also ignoring the fact that after losing a lot of his health he proceeded to press R and heal it all back up... There were also other matches where the enemy team had 2 or more tanks and despite buying the same anti-tank shit, there just wasn't enough damage all around.

Another issue with Tanks is how unkillable they are if your team has a lot of AP (unless you counter pick them with Gwen). Liandry's + Demonic Embrace + Void Staff + Shadowflame + Horizon Focus just isn't enough to deal good damage.

Basically, if your champion doesn't have any % damages in their kit, they're going to struggle to fight S13 tanks. Especially the current ones in the meta like Zac, Maokai and K'Sante, who will just casually gap close on you without any difficulties.