Why do tanks just leave at the start of duties?

[Doozer]

There was a common tactic in WoW where a DPS would queue with a tank or healer friend for something that has a stupidly long queue, only to have the tank/healer friend drop from the group the moment they got the DPS shortlisted through the queue. Hell, some tanks/healers actually SOLD this service to people for ingame gold.

Could be that?

But then that friend is screwed and left without a tank. When I queue with my DPS friend as a healer or tank, I just do the duty, because it's the nice thing to do.