I've played tank roles in MMO's for going on 10+ Years. Everquest as a warrior tank clearing all content when it was still considered an achievement to kill something onto WoW as a pally Tank. You're going to run into people who role tanks because of the fast queues, easy group pickups not because they enjoy the class.

Dungeon Finders, Queues, etc has been a downfall of the tank class for this reason. I had at least a handful of groups asking me to tank for them the second I logged on because I had proven myself to countless people that I was good tank. By no means the best but a good solid tank. It was personal, good tanks had groups waiting (same with healers of course), bad tanks couldn't get a group to save their life.
My point is, when you find a tank that's a good tank. Friend them. Ask them, would you be open to me messaging you from time to time to put a group together. If you want to stop taking chances of fail tanks, remember the good ones and make a friend.