I think it's just a design choice. The dragonborn being the default race doesn't mean it's also the canon race. Same as the class: while being a sorcerer kinda makes sense, saying that you gain your powers from Bhaal himself as a gift or bloodline privilege, it still doesn't fit a murder hobo like the Urge to fight from a safe distance hurling fireballs. They just had to go with a specific look for the artwork and honestly the white dragonborn sorcerer just looks really cool.
