Well like, most people are happy to repeat bahamuts coil because it drops items you want and it's a fun challenge.
People don't like to repeat say, Wanderer's Palace or AK because the content is easy, there is no challenge, the rewards are weak, and there are only so many times a demon wall is fun.

Dungeons don't work well in this game because you know exactly how many you need to do to get all your items. There is no "maybe I'll get a mount" or "hopefully that lvl 80 body will drop!". It's all garbage loot, and with how easy it is to get ilvl 70/80 gear you're almost better off spamming CT. Since 1 CT run is almost a piece of lvl 70 armor. Heck, for 50k/piece you can buy crafted HQ lvl 70 items.