Building for Tanks Sucks
Built Armor? Great! Won't do you much good against all the true and mixed damage enemy champions have in their kits.
But maybe the AD enemies don't get fed and ahead and you dead...Maybe, right?
But then you have the fed AP mid lane enemy and you realize that the armor you built to counter 4/5 of the enemy team amounts to a burning sack of dog crap.
So maybe you try to get some Magic Resist with what little gold you don't have because it was all wisely invested in armor.
Too bad for you, because, at that point, the fed AP guy has invested 7k+ gold in their full AP build; so, even if you somehow managed to finish that Spirit Visage or Adaptive Helm or Abyssal Mask; they'll still delete your HP bar as quickly as your carry gets one or maybe two shotted.
And let's assume that your team somehow turns this bad scenario around and you do get a couple MR items. Fantastic! You can now survive one more second against that AP monster.
But wait a minute! It's late game. Who allowed the Yi/Yasuo/Twitch/Vayne/Nasus/etc free farm when they went 0/10 by minute 15 of the game? Well, guess what? Sucks to be you again because even though you died maybe less than 5 times in a 30+ minute match. Those 0/10 scaling carries now have the power to turn you into their personal wad of toilet paper.
TL;DR ^TITLE!