Excluding the ones from the quest line (one of which I used for Thorn), I've only seen approximately 10 keys across three characters. But I understand why the drop rate is so low: cache keys are one way to get to 400 light, so having the key drop rate higher could trivialize leveling those last 10 light levels. Additionally, if you recall the original specific strike loot was a low drop rate in the first place; I ran Fallen SABER at least two dozen times back in TTK before I saw a Maw on my Warlock, so the key drop rates are roughly the same rarity for the same loot - the only difference is the key gives the player a small bit of control as to which chest to open.