The real reason is (Harsh Version):

Both Tanks and Healers are artificial roles with no point nor purpose. They are forced into the group via un-avoidable damage and are only ever as useful as they are mandatory. That's also why they end up becoming DPS as soon as Mitigation/Healing requirements are met - They are innately useless. That's also why they need Damage abilities for solo play - Because they are useless at their core. Damage is what matters, all that matters. And people subconsciously pick on, hence they prefer playing DPS.