Because tank low-level skills do not generate enmity fast enough in proportion to the amount of damage (and therefore enmity) generated by dps (or healing by healers). Also because a level-synced player will rip hate off a lower-level, lesser-geared tank (almost) no matter what, unless they purposely slow down and watch their own aggro.

And because a tank is expected to lead the way in a dungeon and know the mechanics even if it's his first time there.