Why is class/build balance so difficult to create?

While I agree with this in spirit, The Istar Mithrandir aka Gandalf used both a longsword and a staff in these battles!

Given that my first exposure to the idea of a mage/wizard was this character who wielded both staff and sword; It has always annoyed me that most games don’t allow mage classes to use swords. I was Stoked when I saw Sorc could now use a sword (also loved battle mage in DA Origins!) … now we just need to be able to equip sword and staff! OR a new dual-wield mage class in the future that can 0.o