People don't "not tank" because of lack of damage. People don't tank because the tank has more responsibility than everyone else. They have to set the pace, know the pulls, know the mechanics, corral and place the mobs, etc. etc. And if they do any of these things to the dissatisfaction of any of the other players in their group, party chat will be filled with an assortment of comments ranging from passive-aggressive comments to outright harassment.
Tanks know that if there is a wipe, they and the healer have a 50%+ chance to be blamed for the wipe regardless of the fact of whether it was their fault.
So that's the story of why their are few tanks, and most pug tanks who will go through that are a mix of very arrogant and/or just don't care what anyone thinks. Most won't bother, and they'll just find something to do while sitting in queue as DPS or heals.
- - - Updated - - -
Pretty much this. I don't get why people whine about it when they know they aren't willing to do it. Roll a tank and you'll never whine about lack of tanks again, because you'll be in high demand.