An autopilot is a system used to control the trajectory of a vehicle without constant 'hands-on' control by a human operator being required. Autopilots do not replace a human operator, but assist them in controlling the vehicle, allowing them to focus on broader aspects of operation, such as monitoring the trajectory, weather and systems.
Except Tesla's system is designed such that it assumes your hands are on the wheel at all times, and you really can't take your eyes off the road for doing things like checking the weather. In a plane you most likely have plenty of time to do something if the auto pilot fails. Probably upwards of 1 minute unless you are landing or taking off. In a car, you're lucky if you have 3 seconds to notice a problem before correcting it because the other vehicles and obstacles are so close.