Wouldn't making AP champions tank killers make more sense from a design/balance prospective?
Tank killers are generally such by virtue of having a large amount of % HP damage and/or true damage. While there are a few AP champions which fall into this category, 
most don't. Generally it's put onto AD champions, mostly fighters and a few marksmen. This is the way in which riot has decided to balance their game.
Though, I'm really at a loss for why it'd be better to make AD, the stat which has implicit flat damage scaling, the stat which is often paired with % max HP damage. Where as you could easily manipulate the amount of flat damage output a mage or AP fighter has, because their only real source of damage is through their abilities and they have no implicit scaling with their primary damage stat.
So if you were to design a mage, which was designed to have their most of their abilities deal % HP damage, then they'd have a champion which could kill a tank just as fast as they could kill a squishy target. By balancing the champion so that they have a longer time to kill on the squishy than someone of equal power with more flat damage scaling, you'd have a champion which kills a tank faster, but a squishy slower than flat damage. Making a fair trade to gain tank killing power.
Though translate this to an AD champion and you run into a problem. AD champions get their implicit 100% (excluding kalista) scaling with AD via their auto attacks. So you have a champion with % HP damage, becoming an effective tank killer, but they really haven't lost much of their squishy killing power due to the flat damage output of their auto attacks. Meaning you're making the margin for error much smaller to balance them against champion which only deal flat damage. So adding % HP damage to AD champions really doesn't leave much design space when the tank killers haven't sacrificed any of their killing power towards less tanky enemies to gain that advantage.
To get a better idea of what i'm talking about, look at the two items which deal % HP damage, and their stats.
Liandry's gives AP, which doesn't necessarily mean that the champion buying needs to gain a substantial amount of killing power towards squishy targets. They can, and often do, but this is at the discretion of riot.
Where as Bork has it's flat AD and atk spd, paired with % HP damage, meaning that buying this "anti-tank" item is also going to **always **increase your squishy killing power too.
I made a similar post yesterday and for some reason people interpreted it as "i have no idea that fighters, juggernauts, and marksmen are intended to be the tank killers". So I'm adding this into the main post as a disclaimer to all of those who appear to be infected by brainslugs. I'm well aware of this design decision from riot, i'm questioning riot's judgement on this decision because it seems to contrary to their same desire of wanting to increase the viable design space for the game.
