Depends on the tank in question, but generally, I find the defining role of tanks to be their disruptive abilities. Poppy counters dashes, knocks enemies out of the fight, and makes you fear walls. Maokai can zone entire teams with his Ultimate. Rammus and Malphite are great big fuck yous to enemy ADs. Most of the time, these mechanics (or key parts of the champion's functionality as a whole) are implemented in a way that favors surviving for a long time, which combined with a mostly melee nature requires Tankiness. However, defining a Tank purely on their ability to be meatshields puts them in a weird catch all box with Juggernauts and some Dive Fighters with is just wrong for a wide variety of reasons. The exact specifics on the difference between Tanks and Fighters is its own discussion but you need to get it through your head that Tankiness is the way you achieve your Tank's goal, but Tankiness is not the goal your Tank is trying to achieve. If Braum is alive for the entire duration of his Unbreakable, that's a major boon in big teamfights and the extra bulk helps when he uses Stand Behind Me, but the goal isn't to keep Braum alive. It's to keep his Unbreakable up and moving in the middle of a fight to stop projectiles first and foremost, and everything else, including his Tankiness, is a means of achieving that goal.
You're right though in that a lot of Tank items don't really enable them to do their jobs so much more as a lot of those items' functions are just balls of stats. Funny enough though the Tank items that are good at enabling certain Tanks needed to be nerfed since they were also very universally powerful, see Righteous Glory. The problem here though designwise is two fold. Riot decided a while ago that they wanted Tank itemization to be streamlined in terms of components to finished items since they were a more reactive purchase than say a Carry's core items, which is why all armor items start from Cloth Armor but not all AD items start from Long Sword. What this means though is that the power budget of many Tank items are sacrificed for flexibility; great if you rushed a bunch of Cloth Armor and Ruby Crystals early to survive laning phase but terrible if your end goal is to be a gigantic end game wall because now a lot of Tank items are slot inefficient. The second problem stems in the fact that a lot of Tank designs are very self contained; what works for Shen works differently for Galio which won't work at all on Gnar. This leads to very overly generically strong items like Righteous Glory in its prime, stat stick items like most of what we see today, or hyper niche items like Zz'Rot Portal or old Thornmail on Malphite and Rammus. Tank itemization wrote itself into a bind.
Anyway, what you need to take away is to recognize what your Wardens (defensive Tanks) are designed to stop and who your Vanguards (offensive tanks) are designed to step on. Notice Poppy's utility against hyper mobile teams, notice Shen's ability to unlock reliable splitpushing without boning your teammates, single out and absolutely destroy the enemy team's ADC as Rammus, but for the love of god stop thinking of Tanks as meatshields. They have abilities, really good abilities, that must be applied correctly based on the situation, based on the tank. Figure that out and you figure out Tanks.
