That's something I've wondered about. Hit and runs are very common, of course--emphasis on run. The impulse is to step on the gas and get away.
I've heard of a couple of horrible cases where someone was hit, catapulted through the windshield, or entangled in the undercarriage, and carried off, because the driver was too incapacitated to act sanely.
I've actually never heard of a missing person case that was solved by proving that someone had accidentally hit the person, and then had scooped up the body to take somewhere and hide it. Can anyone name a case like that?