You don't understand tanks at all man.
It has to do with their role in a fight.
Look at a Warrior in WoW. Yes, they can be specced to be a tank, but almost always they will be speced as a melee damage dealer with some AoE capacities. Dedicated classes like Paladins or Deathknights however, are much more commonly tanks, and especially main tanks, as they have the crowd controls and especially TAUNTS In standard MMORPG terminology, if you do not have a method of taunt or forced aggro, then quite simply, YOU ARE NOT AND NEVER CAN BE A TANK. PERIOD.
For that matter, even off-tanks cannot function as true main tanks unless they are massively over leveled/over geared, as they would need to outdamge DPS classes to maintain aggro, and still would need the defensive gear to function as a main tank without the resistances, immunities, and other massive defensive cooldowns like party-wide invulnerability. The same is true in League. Garen or Darius ARE NOT FUCKING TANKS at BEST they can be described as off-tanks, but even then they are functionally and fundamentally different from a main tank like Leona or Braum, and do not have the CC to effectively initiate fights or peel off divers/assassins.
In actuality, Garen and Darius are juggernauts, target and mobility restricted melee carries. They, like the aforementioned Warrior, are melee DPS classes-- however, to survive being in that melee with all the AoE and ranged damage they would take just wading into the fight, they need extra sustain or bulk (note, Darius has no extra bulk, and Garen no in combat sustain, they each only have one piece to the puzzle) to do so.
The mistake, is fools like you then claim this classifies them as a tank, in completely and utter obliviousness to how they actually play and what role in combat they actually perform, and the skills and abilities that they lack. If someone's main focus is damage dealing, they ain't a goddamn tank. If their job is to take attacks for allies, shield others, CC and prevent their opponents from being able to act, or especially damage their allies, then they are a tank.
Melee classes in MMORPGs are tankier than ranged pretty much categorically. Does that make every melee character a tank, because they have bulk? wtf no. That doesn't even make sense, and even applied to any other game than League, still wouldn't make sense unless you only had 2 different classes and no subclasses/specs of 1 ranged and 1 melee.
Look at the tools champions have available to them, and what role they fulfill in fights, more than looking at their stats or item builds.