If you think of it VGT's may have kind of been outsourced to their respective manufacturer's design teams. They are rather smart product placement and marketing tools requiring a singular investment. Deeper/direct collaboration with the involved departments may eventually lead to better understanding of game model specific requirements and possibly(?) serve as a basis for future streamlining between manufacturers CAD and Gran Turismo.
Maybe. Maybe not. Either way a CAD model will always just be a base to start on their own 3D model, there is no magic 'ta da' button to just generate a game model from a CAD model and I really can't see PD 'let' someone else create their car models, we know how anal and protective they are. They want everything done in house.