Armor Vs MR/Mages vs ADC
This is pretty simple. You can get over 500 armor in one game very easily. But what about MR? Thornmail and Randiuns alone are EXTREMELY good items at countering and screwing over melee/ADCs.
So where are my AMAZING MR items? Thornmail gives 100 armor. There isn't a SINGLE MR item in the game that gives that much. If we lok at raw numbers, that doesn't make ANY SENSE. AP can go past 1 THOUSAND. Most AD (Ignoring Jhin) don't go past 350. So why can we stack so much Armor, on items that give other benefits, and yet not have any good MR items against mages? All of the typically picked up "MR" items have glaring downsides: Edge of Night: Only has 35 MR. Sure, it can suck up a SINGLE spell! Then what? It's good for quick fights, not prolonged ones. Banshee's Veil: Has the highest MR of any item in the game at 70. And has a cooldown almost matching Edge of Night on the spell block (5 seconds less, woohoo!) And what do you get over Edge of Night? 300 WHOLE hp! AMAZING! Abyssal Scepter: Worthless on Bruisers/ADCs. Picked up on few Mages. The MR isn't what this is picked for. Zz'Rot Portal: Great! You picked up an item to push a lane. And it has 55 MR. Again, not what this is picked up for. MoM: While toted as THE MR item for melee and ADCs, the shield is pitiful endgame and the MR isn't much to speak of. To show the math: 6 Thornmails gives (Without Armor Runes/Talents) 698 armor on a Aatrox. 87% damage mitigation. 6 Banshee's Veils gives only 473 MR. That's 83% damage reduction. Yes, any one of these MR items can take you from 20-30% Magic Resist to 40-55%. But with most mages STILL melting people faster than a fed Talon, call me crazy, but I think that's a sign we need new MR items.
WHY this is a "problem"? Mages stack AP WHILE stacking Armor/MR/HP/Mana. All the good AP items gives GOOD stats to go with them. And most mages pick up Zhonya's, which not only provides them with 3 seconds of invulnerability, but 45 armor, CDR AND a hefty chunk of AP.
Name me one ADC item that does even half of that. Frozen Mallet/Sterak's/Cleaver are all WEIRD items on ADCs, and typically lower your output MASSIVELY. AND they are the only items that have both AD and HP on them. What about Armor and AD? Not possible. With the removal of Atma's Impaler several seasons ago, all AD/Armor items ceased to exist.
My point? Mages typically end the game with 2.6kish HP, good amounts of MR/Armor/Mana and enough damage to oneshot ANY ADC in the game. Mages have it nice.