In the beginning, the developers created nine classes. Warriors were the tanks, Priests were the healers, Rogues were melee, Magi were casters.
The other five were hybrids, each designed with totally unique perks that contributed to group play and made up for their lack of a singular strength in tanking or healing or damage per second.
But one day a Paladin rose up and declared;
"OMGWTF Y CAN ROUGE DO MORE DPS THAN ME FFS IMBALANCEW"
despite the fact that he had eight times the armour, the ability to heal himself, raid buffs, raid utility and the strongest defensive cooldowns in the game.
More hybrids rose up, unsatisfied with their unique class benefits, to join the throng, and at last the developers realised that nobody cared how many defensive abilities or crowd controls or unique and powerful buffs they had. A benefit which cannot be measured cannot be boasted.
And so the developers declared that all classes would do equal damage, all healers would heal equal damage, and all tanks would take equal damage, since these were the only things that mattered.
And the hybrids rejoiced, for now the jack-of-all-trades could master each, and all classes would be equals-- little matter that they were somewhat more equal than the rest.