Yeah, I kind of wondered if technical limitations might have been part of the reasoning. As I played the game, I couldn't help shake the feeling that the developers had originally intended you to do more as a Dragon.
But, mainly, it kind of seems out-of-story to have the player character become a dragon, and then never fight any enemies (except a few flying demons and Svadilfari) as a dragon. Other than transport, it would seem the player being a dragon only serves to explain why you are able to attain such high levels of character development, but your actual power is whatever magic or fighting skills you employ in human form, mostly (although, I have to confess that when fighting in some areas, especially the Flying Fortresses, I will sometimes morph to dragon form to either get away from enemies if I'm close to death, or simply to regenerate my magic and to heal using the Dragon Spirit skill; that definitely makes you a lot more dangerous to the Black Ring).
Maybe this new Dragon Commander strategy game they're cooking up will allow more ability to engage ground targets as a dragon.