Tanking doesn't really carry more responsibility than any other job. It's just most NOTICEABLE. If a DPS uses the wrong ability, with a few exceptions in the late game, it's hard to tell. If a healer uses the wrong heal, the tank might die or it might be confused with the tank taking some big damage. If the tank loses hate, well, it's pretty obvious.

Without a tank, the group can't survive the boss damage. Without a healer, the tank (and thus, the group) can't survive the boss damage. Without DPS, the group can't kill the boss and it either takes forever or the group wipes to attrition/enrages/more opportunity for mistakes. That's why they call it the trinity folks; 'cuz you need all three.