Automatic ship out can cause conflicts when one vehicle wants out while another wants in. If we could make it work, it'd e a great feature. Will be more complicated than just automatic loading, though.
One way to do it would probably be to wait with the ship out until the entrance of the garage is free. But that has to be detected.
Furthermore, if automatic ship out is added, a switch will be needed to turn it on/off per garage. This is because sometimes players want to keep their units in garages.