The whole dang team could be getting nuked by an ap champ 17/3 
(for example) and the average mr for 4/5 of the team is gonna be in the mid 40-50s, but have 2 armor items each. I know mr items aren't cool guy damage optimal, but you can't do damage if you get exploded by a wizard.
Cuz magic pen is such a strong stat, and just having magic pen boots and void staff makes all that mr go to complete waste Imo, it's better to just go full dmg to nuke the mage that is oneshoting, since he will oneshot you no matter what
mr stinks
stinks
all mr items are stinky
because building most MR items immediately puts you behind the rest of the game.
Nothing can be done if said wizard already starts taking a dump on you with high base damages. Also MR items just generally suck. There's less of them around, and most would be purchased on a tank, which then drives the point home the item sucks, so then you come to the current situation. Spirit, Wits in some cases, Qss (full upgrade) and Maw are the only really good MR items, but a simple 2800 just shuts them down completely (which knowing mages, isn't hard to farm and obtain)
I could be a Garen with 180 MR, and that Ori or Ryze will still deal 400+ damage.
Because damage is so obscene that any resistances is pointless to build. Not to mention it often impedes damage stats that everyone loves, right now.
Mix of ad also effecting towers, minions, jungle, ad being universal, hp being effective against mages but not against ad, spells can be dodged autos can't. Armor items have the better passives, movespeed buff or damage or slow. Spirit Visage only competes if the champion has built in sustain and other two tank mr items have low hp for no reason.
