Because this class homogenization thing sucks. Warlocks are pure dmg dealers, thats how it is. There must be some limits in giving players CHOICE. That would be a precedent for making hunters shooting healing arrows, frost mage tanks etc. I suppose you would like that, but I would definitely not. Besides, the only reason for wanting this is that you would feel as something special (caster tank). All of us want this, thats why are we transmogrifiing sets noone has etc. The point is, your need would in the end cause everyone being the same.