We have a Thornmail but how come no Mirror Shield?

Probably because

A. Skills shots

B. It's less consistent

C. Mages don't really have a way to heal back from damage

Presses R and hits tab to see enemy builds : "And I just 1 shot myself"

yeah NO. lemme explain why its a bad idea friend. see auto attacks, you can actually avoid doing. adcs understand that, and will try to poke you down with abilities if they have to. now here is my point. ready? . at some point in the game, he will have to decide if he wants to die to that ap thorn mail person. there would be no counter play to it unless you forced aps to go gunblade every game, making the item kinda useless in the end anyways. its just not a fair concept. especially if it was like, a mundo, with thorn and ap thorn mail... you couldnt kill him and he'd kill you by standing still.

tldr; bad idea.

Thornmail is easily countered by building lifesteal. A magic version of thornmail would have no counterplay as there are no items with spell vamp in the game anymore.

This is a neat idea. One issue with it is that thornmail is generally bought to try and counter auto-attack based Champions that life steal a lot. I do think it would be nice if tanks could buy an item to help against cosistant damage AP champs like Cassio in the vein of Randuins or Frozen Heart though. Something that made it so they could spam ablilties so hard like the other make ADC's auto less.

Because then Victor would kill himself with 1 spell rotation...

Well technically he would still kill your entire team with that rotation.

Thornmail needs a buff. Master yi can solo a rammus with one lifesteal item. Or maybe I equipped the armor wrong? Actually, it seems anything except maybe the adc has been able to solo my rammus with thornmail without taking damage. Adcs get melted tho, except for graves....

It would hard counter all DoT and AoE champions. Have at least 3 people on your team buy it and the fight becomes a 5v4 against a team with AoE Magic Damage characters.

Yeah let's make tanks even more broken!

wasn't there one , well kinda.... on that map that no one remembers? hmm... what was it called...

There are more items that grant lifesteal for melee attacks than spell vamp for AP spells, as others have mentioned, so countering this would be nearly impossible. Plus it's a bit more important to have an item to counter unavoidable melee AA's than it is dodgeable ranged skillshots and the like, the latter being a niche to fill with champion abilities.