They've already mentioned a few times that tank and healer damage are not considered for balancing the content. Gordias was a special case, I've heard they didn't have enough time to test it and fine-tune it and that's why the dps checks were insane.

But otherwise the reasons why tanks and healers aim for bigger numbers can be:
1.- As a personal goal they aim to do better and they're already fulfilling their role well.
2.- Many dps in this game are BAD and they don't have any interest in improving, so when the group as a whole meets the enrage or adds don't die fast enough, etc. it gives the feeling that they HAVE to contribute to the group's numbers too.
3.- "Elitists". If they CAN choose between you and another tank/healer who's pulling bigger numbers guess who they're gonna choose.