Fire Mage has the skill “Critical Mass”, which increases critical strike chance.
Frost Mage has the skill “Winter’s Blessing”, which increases haste.
Why doesn’t Arcane Mage have a skill that increases mastery?
Without a skill that increases mastery, we scale much worse than the other Mage specs.
A similar situation happened with “Orb Barrage” before it was added as the Arcane equivalent of “Firefall” for Fire Mage or “Cold Front” for Frost Mage.