Because every Dex-Mage out there has prefered the Avian bird over dex mage. Dex mage is a weak int mage and avian bird knock off, so that's why its not too common.
You should try a Avian bird, you will most likely enjoy it more then dex mage, if you enjoy the dps side of things.
Also to add onto what Royce said, as a Dex Mage, you may be hitting 500's-800's damage, but you are hitting very weak for your spells. A Int mage can hit up to 800 damage per enemy with a fire blast spell. All aoe damage spells combined, easily thousands of damage more per second then a dex mage.
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