Most of the automation efforts revolve around "drive on the highway from point A to point B" scenario. If you have a container that's that simple, rail is much more cost-effective.
Trucking has fairly complex loading/unloading requirements, with scenarios such as "haul 8 vehicles, unload them at 8 different addresses", "haul a load of tomatoes and lettuce, do 5 drops at In'n'Out locations, 1 at Safeway, 1 at Albertsons" fairly common. There's an understandable lack of trust in the system, where the trucker is also responsible for ensuring the staff on the recipient side is not unloading (i.e. stealing) too much or helping themselves into another customer's load.
There's also "stop and wait your turn at weight stations", "stop to refuel" as well as "stop at random checkpoints when crossing state borders" scenarios that seem to require just way too much of human involvement.