Why is armor completely fucking useless? I can build 150 MR and negate almost all magic damage, but i can have 300 armor and an ADC with 200 AD no last whisperer fucks me. Explain this shitty fucking system so i quit getting my fucking ass raped in a game as a tank and feel completely fucking useless k thanks.
That's called "sensationalism".
"tanks" never existed in this game. either you have abilities to block/prevent damage or you cant "tank". tanking never involves soaking damage. you can only tank for your team if you have abilities that enable your champ to waste the enemies time without dying but having too much cc/damage to be fully ignored.
for example, if you are maokai and you jump on a big jinx, you will simply die instantly. but if lets say an enemy gets caught (not by you) or jumps onto your carries, you dont jump onto theirs but you THREATEN to jump onto them so jinx backs off for 1 second or so and you back off too so she effectively wastes time in which she doesnt auto attack. if she gets too close to you by herself (not by you jumping onto her or walking into her team) you can arcane-smash her, slow her with sapling and walk away a bit.
if they want to kill you, they must really commit alot of cooldowns onto you because you are standing far enough away so that not everyone can just unload on you and if the cc ends, you can arcane smash and walk away a few steps which makes you very hard to kill and chances are your enemy who jumped into your team died much faster because he was in the middle of them. you now effectively tanked for your team.
lets say you are lissandra top and you have a very high damage team with like anivia mid. you can still "tank" for your team not by going in and ulting yourself but by dancing along the edge of the enenmies range, shooting some Qs and threatening to go in with your claw. this can make them back off for a few steps if done right or make them chase you for a few steps which effectively wastes their time aka "tanks". as long as you dont risk getting killed instantly, you can win the fight by doing so. and if they ever go on your team decisively, you can always still go in because you have gapclosers and alot of damage so they just lost the fight by ignoring you.
also, some thing about armor items: stacking defensive items has diminishing returns so for every armor item you get over the first, you get less value out of it. if she gets a damage item instead, she effectively has more value from her items than you. also, offensive items are just stronger by themselves (but also more expensive) so if both parties have 0 items, tanks are pretty tanky. if both parties have 1 similar-cost item (rapidfire gun vs deadmans plate) the tanky item wins BIG time but with every item they get, he falls behind more and more and at full build, if you get 6 tank items and the other one gets 6 damage items, you get melted instantly because his items are stronger and stack much better.
this means generally its not worth to buy more than one defensive item of a type (1 armor, 1 mr) and only if their AD damage is really insane, add a thornmail to the mix.
other than that, unless you are fed, you are just wasting too much gold.
the best build to be as tanky as possible while still being a threat is to mix semi-offensive items like black cleaver, titanic hydra, trinity force and whatnot into your build. for example, as the best shyvana player euw i can tell you that last season, building devourer + botrk was actually extremely weak. the best build on shyvana by far was devourer + titanic hydra because of how tanky and bursty it made you. much more dangerous than a glass-cannon.
even tho this isnt the best build anymore, the general concept still holds true on almost all melee champions. being a bruiser is the strongest combination of items because defensive items are cheap and strong but you cant effectively stack them so ideally only buy 1 defensive item of a type (armor or mr) and pick a target you gonna isolate and duel. for example you buy black cleaver deadmans youmus and you isolate that khazix jungle on the enemy team with sweeper and pinkwards, he cant kill you by himself so he will either eat your full combo which kills him instantly at black cleaver alone or he will get help from his team which makes your fight 2on1 but your teams fight 3on4 which they can effectively win. and if you know how to play a 2on1, you can carry your team with that move even tho you didnt pentakill.
because autos are way more frequent than spells and if you're facing some1 like vayne. then no amount of armor or health will be useful. she will kill you just as fast at any point of the game
Because that ADC also has a ton of AS and crit. And their damage is not only point and click, but also resource free (except 
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