It's a matter of responsability, pacing choice, mistakes that will impact the whole group and that no other player will be able to deal with.

A dps does shit ? Well he dies, and the team continue even if he is dead.

The healer is on a same duty basis : You screw, everyone dies. The difference here is the role is mainly reactive and while a good healer will have to be reactive and pro active, he don't need to and can learn at a slower pace, just keeping everyone alive and focusing on this duty.

The tank on his side, is responsible on the pace , his own survival which is essential to the survival of the whole team, and threat which is fought against the DPS Performances of the two DPS présents. Pull a pack of three and get two BLM single target focusing two different ennemies, and you will get the whole "against" part.

It's no randomness that the tank are few and fewer. They endoss stress, criticism and heavy responsabilities, and while anyone can survivre a dungeon with a healer that is slow or healing focused, or a DPS that focus one target, avoid zoning or just play badly, a tank that is poorly played will end up in the worst experience you can have.

Then, if tanks are the easiest to play, why are they so few ?