Doesn't it present the same problem? Yes they can hit you...but the enemy is still going to recognize that you're not dying from those hits. Ultimately to me that reads as the same thing: they're trying to kill you and it's not working and there are much more killable targets in range.

As much as D&D has traditionally promoted the "holy trinity" only 4E ever made it functional beyond requiring a healer. Even in MMOs, the "hard to hit and hard to kill" portion of tanking only really functions against heavy-hitting "bosses". For crowd-control, almost all the classes have AOE and group-management abilities. After the "failure" of 4E, you've never going to get true "tanking" mechanics in D&D.

Rather than attempting to build a low-AC, high-HP, damage-reducing, self-healing "tank", it'd be simpler to include "zone control" feats or class features. A "Combat Reflexes, but for tanks" ability that increases the number of AOOs you get, or a "taunt" mechanic that punishes enemies for attacking enemies other than the tank would be simpler to achieve the end goal of an MMO-style "tank".

Quite frankly, D&D enemies aren't dumb-ai controlled morons either. Even relatively stupid creatures can figure out that their attacks aren't having an effect on a given target.

On another note, 5E runs on something of an "arrow to the knee" function. Players tend to be hard to hit, but have low HP. Monsters are the opposite, with high HP and low AC. This is also a rewarding mechanic that lets players feel "successful" (by killing monsters and not dying). A low levels, high AC is all a "tank" really has to protect themselves from the natural randomness of the game.

TLDR: Enemies don't target you because you're easy to hit. They target you because you're easy to kill. Hitting a target is meaningless if it isn't dying. "Tanks" in D&D need better crowd control, plain and simple. From Warlord-esque "shouts" or magical auras to compel enemies to attack them, to straight-up "taunt" effects, magical or otherwise.