Dungeon keys can only drop after the point in the campaign at which they exit. This allows subsequent characters to utilize them as campaign skips, while preventing first playthroughs from being RNG in if you get a campaign skip or not. While T1 Lightless Arbor starts at level “22”, it’s not a particularly easy task at that level. This is as it’s intended to be for more experienced players that have already beaten the game once to utilize as a skip (though we have been discussing the exact difficulty balance of this).
Each dungeon also has multiple tiers, scaling all the way to endgame. In the case of Lightless Arbor, once you complete the campaign, it automatically unlocks a higher level T2 version of the dungeon that will be closer to your level.