Because they aren't nice. They are cool exactly once and then a headache every other time thereafter. I do not understand this sentiment. What is so exciting about your tank getting lost and taking a wrong turn in totorak and your reward is... extra boring trash in one of the worst dungeons in the game? And practically, all those side paths may as well not exist because people will figure out the fastest means through a place and that trickles down pretty quick through the games playerbase. People had Qarn, Darkhold, Totorak and Cutters cry figured out by the end of the first month of ARR launch and you'd almost never blunder down one of those side routes anyway, at which point they don't serve any purpose.

Same deal with the Qarn puzzle. Like I think it's neat but the reality isn't 4 sprouts staring at the scales intently, trying to solve the puzzle, it's someone checking a wiki for the answer or someone that has already done it and knows the answer. The little mystique these things grant on first encountering them is not worth the tedium they produce on repeat encounters in the future.
Majority of people don't "explore" deep dungeons. They're just used as a leveling tool to get exp on DoW/DoM jobs. As such it's basically rush rush rush to get to the boss asap.
So what I'm getting from this whole conversation is: for content in FF14 to be capable of supporting quality exploration... it needs to both be decoupled from levelling and needs to (somehow) discourage players from running it repeatedly (thus reducing the desire for it to be streamlined).

What kind of content like this would they be able to make?