1 - You were playing Zilean. Regardless of what you built, he has low base stats as he's a squishy, immobile mage who is designed to AP support/mid, not off-tank or buy more than one resistance item in general.
2 - You bought Remnant of the Watchers 29 minutes into the game, Merc Treads 30 minutes into the game, and Ruby Crystals/Haunting Guise about 33 minutes into the game. Basically for the majority of this match you had jack in terms of resistances and by the time you did buy them it was too late because Ryze, Twisted Fate, and Zyra all had their items or were fed.
3 - While I do feel that MR should be more available and have 1-3 extra choices, your itemization and rune setup were unoptimized or just plain bad. With all the stat shards, items, and base resistances that Zilean has, you finished that game with 134 Magic Resist.
Adaptive Helm - 55 MR
Merc Treads - 25 MR
Two Magic Resist Stat Shards - 16 MR
Oblivion Orb - ~~ 200 HP ~~
Haunting Guise - ~~ 200 HP ~~
Remnant of the Watchers - ~~ 200 HP ~~
Grand total:
1,793 base HP + 600 bonus HP = ~~ 2,393 total HP~~
38 base MR + 96 bonus MR = 134 total MR
That's incredibly low and you had better items available to you, namely Bashee's Veil, Athene's Unholy Grail, and Abyssal Mask which you should have built way earlier, again, not over halfway into the match after people were already strong/built.
#TL;DR: If you want to complain about Magic Resist itemization, don't try to use this game to support it.