It's pretty simple really.

All the bad/mediocre tanks have quit tanking altogether, probably due to all the abuse they get which makes it difficult for them to actually learn their role.

All the good tanks are already in an FC, meaning they don't use the duty finder when doing dungeon runs.

The only time I use the duty finder when I'm on my paladin is when I'm feeling bored and want to carry people through ifrit and garuda HM since I can both main and off tank. Other than that I just ask for DDs from my FC or my friend list since at least then I know that I'm getting competent people in my party.