While technically an autopilot might sometimes be as simple as maintaining a heading, the actual practical consequence is quite different for a boat or an aircraft than for a car. There is simply not much to crash into when you're in the air or open water. The result is that a much simpler mechanism is required to achieve the same result for the pilot.

When I worked on unmanned vehicles, you could have one operator control multiple speedboats because you typically had minutes to avoid collisions. Splitting attention would not be feasible with a car on cruise control, because you are never more than a few seconds away from crashing into something solid.