Because edumacation.
Qarn is one of, if not the most skill and positioning sensitive dungeons in the game. If you can master Qarn, many of the issues encountered in the harder endgame content can go away for you.
Wanderer's Palace is the quintessential "My First Endgame" dungeon. It can be beaten by a team in all AF / AH gear without much issue if they all work together as they're supposed to.
Running these dungeons in DF is a perfect way for the veterans and higher end players to teach the newer faces and first timers the tricks and gear they will need for later places like Coil. The key to doing that successfully is to be nice to people. We're all familiar with the defensiveness that pops up so quickly when people start getting called on mistakes or bad gear choices. It leads almost inevitably to ragequitters and arguments. But what I've found is that if you treat the other player not like a stranger, but like a friend, and you talk to them like a friend, make suggestions, and explain why the suggestion is better than what they're doing, that the overwhelming majority of even bad players will listen.
My FC has started making it a point to no non-speed WP runs nightly, going in teams of two through the DF, specifically to help other players master the dungeon and improve their skills and gear. Plus we get swag out of the deal ourselves and have a good time killing Tonberry King.
The only way to make the community better is to roll up your sleeves and work to help others improve themselves. That's not accomplished by ranting on the forums about how terrible they are. A few will always be beyond hope ("Weed Fourtwenty the Bikini-only BLM" is probably not going to be taking your advice no matter how nicely you present it) but the vast majority of weaker players can come up to par with a bit of kindness and patient effort on the parts of higher levels.
I've been running Sastashi and Toto-Rak solo for weeks now for no purpose whatsoever other than to teach new players all the stuff not in the manual. Even by the ends of the run the performance improvement have ranged from noticeable to outright amazing. When some of these guys get to endgame, they'll pay it forward and the community will continue to improve.
Want FFXIV to be a better place for everyone? Go out and work for it.