Same reason worgen don’t show their human forms on the character select screen.
Except dracthyr models can’t wear armor, unlike worgen. It’s honestly pretty sad and makes me want to not bother playing as one, because your options are:
A) Baked-in NPC armor forever. Except for belts and shoulderpads. (Except not actually belts, just belt buckles.)
or
B) Be a horned human/belf, in which case you may as well not be a “dragon” at all.