That was a deliberate design choice, so the dragon (wyvern) form would correspond more to human form (ie arms become wings, rather than wings just forming).
The dragon Patriarch had arms, legs and wings in Divine Divinity. For D2:ED / DKS I don't know if he took a wyvern form to talk to your character, because he wanted a change of pace when he moved to that cave, or Larian just went with a different design.
