It's all a matter of the states and feds being able to keep track via registration of what's on the road. With the documentation from the mfg, the state is able to document the completed car. State titles probably aren't gonna be issued on imcomplete or non-complying autos. When a mfg sends a box of parts they also send appropriate documentation (MSO or C of O or bill of sale) so the vehicle, when completed and in compliance, can qualify for registration and titling.
