It is a design choice. If SE wanted to strip away or make DPS impossible for Tanks / Healers they could. Increase MP on healer's DPS spells to the point where it takes a lot of MP and would run them dry fast. Throw up a damage taken increased % on PLD's and WAR's offensive stances while tuning bosses and even normal monsters to throw out higher consistent damage that would force them to stay in their tanking stance as well as forcing healers to use their abilities to keep their tanks alive instead of passive healing that allows them to damage. But they don't do this because the game wasn't designed with that in mind and was in fact made for people that like casual play and in reality is mirroring ideas that were set into motion years ago in FF 11 when they started easing up on the games difficulty and borrowing ideas from WoW..
Every tank that does damage is a DPS with defensive abilities. Even if their doing 1 damage by just poking, their still adding onto raid damage and if their not taking heavy amounts of damage to require them to stay defensive, their always going to ease up on it to increase their dps. Maybe the next tank made will be one that 100% kills off a tanks damage and instead requires them to sit around casting abilities to negate damage and generating threat by casting spells that don't do almost nothing to bosses other than....generate hate.