Cars need to react on pedestrians, bicycles, children, signs, traffic lights, roads, rain, grip, ice, roadworks....
How would you like to applicate it on ships?
To look at an example of another type of craft...
Airplanes have autopilots which started out primarily to maintain altitude and heading. Eventually more tasks were built in such as following a radio beacon to land planes on runways.
Nowadays GPS is used extensively (we can be pretty sure) to navigate cars and planes and ships. Still there is the occasional situation where a human needs to steer.