Do low ranks not know MR exists?
Because it sure seems that way. I just had a match where the enemy team was Teemo, Lee Sin, Fizz, Lucian, and Morgana. 3/5 are AP. Fizz finished 14/4/3 and Teemo was the most farmed champ in the match with 177 CS. In spite of this, it took 23 minutes for anyone besides me to buy MR. And that was with me trying to hammer home that they needed some in chat.
I've had other matches where people stick to buying armor even when there are no AD threats on the other team. It tilts me more than someone being toxic, to be honest. I can mute them and, as long as nobody else feeds the troll and said troll doesn't hamper us too much, we can still win. But when your team has no idea how to itemize for your opponents, you're gonna have a bad time.