Imagine what would be involved in doing that.
Some analysts think that trucking would be an easy foot in the door for this technology. Freeways present fewer challenges in driving. There could even be dedicated lanes for autonomous trucks. The problem is political. A lot of peoples' livelihood depends on trucking. Those people won't go quietly into the night.
Uber and Lyft want to use this technology for commercial passenger service. They've already disrupted taxis. Dropping drivers would be less of a political hurdle for them. I imagine Uber setting up driverless cars in pool routes like buses. Ride services are also the best hope for completely driver-free tech because no one is going to want to assume driving liability as a passenger.
Completely driverless is where this technology needs to go and I think that taxi/ride-service replacement is the lever. Once that works you'll see people buying cars that don't let you drive.
In planes, when the autopilot disconnects because of some problem there's usually plenty of warning (including a sound like https://www.youtube.com/watch?v=8XxEFFX586k ) and the pilot(s) have some time, seconds or more, to assess the situation and take control. Despite how fast planes fly, the rules of separation and general vastness of airspace make it much easier to smoothly transition control. It may not sound like much time, but consider that in a self-driving car, the latency required to avoid a collision could be in the range of milliseconds. By the time the driver realises (s)he has to take control, it's too late.
Let's also think about the diminishing skills that we'll experience once automation takes over the majority of our driving. Imagine taking 3-4 weeks off from driving and then being asked to take the wheel under a difficult circumstance.
This is also called "living in New York"
I've been railing against the "deadly valley", automatic driving systems that are good enough that the driver stops actively paying attention, but not yet good enough to be trusted unwatched. Tesla's "autopilot" is in that category, or at least it was before they put more restrictions on it. VW is trying to figure out how to operate safely in the deadly valley.
This matters, a lot. Misunderstandings over who's doing what between cockpit automation and pilots are a major problem.[1] There are lots of modes, often too many. This will not work for cars; drivers don't get procedure training, simulator time, and check rides. The number of modes must be small, and it must be very clear to the driver what mode is engaged. VW's slightly retracted steering wheel makes this very explicit. That's clever.
Modeling the design after riding a horse on a loose or tight rein (the proper term is "collected") is interesting. But it's probably too much for a car. As a longtime rider who's had some good horses, I know what that feels like. I had a good Thoroughbred hunter teach me that. Trotting him up a switchback trail, I could either give him a loose rein, and let him work each tight turn his way, or collect him up, bend him around my leg, and control the turn myself. But I had to do one or the other; if I was unclear approaching a tight turn, he'd get through it, but not cleanly. Then he'd toss his head and snort, annoyed that I'd made him screw up. Some people might like that kind of relationship with car automation, but a lot of people won't be able to handle it.
(Military aircraft, though... DARPA is trying to figure this out for the F/A-18E Super Hornet, which is a single-seater with a serious cockpit workload problem. Sometimes the pilot needs to focus on weapons and targeting, and sometimes on flying the aircraft. Doing both at once causes at least one of those tasks to suffer.)
[1] http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.571...