I would say that its most like due to the fact Kunis make the game for PC so didn't even think about this.I suspect the devs did not feel motion control was a viable option for ACC in terms of accuracy.
Kunos did not develop ACC on console. It was developed by another company on behalf of 505Games. D3T I think they are called. Given the incredible amount of work needed on the game after it launched (it was a complete mess when it launched on console) I'm not surprised additional features like motion controls have not even been looked at.I would say that its most like due to the fact Kunis make the game for PC so didn't even think about this.