Why are mages so strong?

Been playing on my alt lately, Idk maybe it's just me but I feel like half my games are just one persons mid is inting and the other mid is 10-0 one shotting the entire enemy team. Sometimes its my mid and we win, sometimes and the enmy mid and we lose. I feel I have no control when their 10-2 annie presses r and kills 3 of my teammates with no counter playe. Oh, let me jump on a mage and kill them before they do that! OH WAIT! Zhonyas. 0 counter play. Oh wait, lemme just buy MR, but wait! The mage standard build has magic pen boots and 2 magic pen items that also provide massive AP.... so unless I build full tank I could never have enough mr to matter. How does a bruiser who needs to duel out team fights compete with a class that can kill 3 ppl in half a second and make themselves immune if u try to stop them.