Why is there such item discrepancy between AD/Armor and AP/MR Items?

Never understood why the game has been around for so long, yet there are so many very useful items with very important stats that are only accessible on one side of the offense/defense spectrum, AD/Armor or AP/MR? The biggest offender is tenacity. Why is going into MR boots the only way to get extra tenacity in game? Why is there no armor based tenacity boosting item? What if the enemy team is near all AD and I want tenacity? Why does Quicksilver Sash have MR? Why not just be that item active and that's it, while building into more than one item for different situations? Why is Zhonyas, an item that just needs removed honestly, only for AP users? Guardian Angel is not the same and not near as good. Zhonyas should just be an AP equivalent of GA. I would just like to see a little more balanced accessibility to such important stats/passives.
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