One point would be that both planes and trains are professionally operated already. That means three of the biggest incentives to automate are missing.
First, they have few human-error crashes; they're both significantly safer than automobiles, so unless automation can do better-than-human disaster recovery (not likely yet) there's minimal safety advantage. You have to outperform well-trained humans instead of random people to save lives.
Second, they don't take much time per traveller to operate. Both often have 100+ passengers per operator, making it relatively cheap to employ skilled human labor. Cars average around 2:1, so there's a lot more time being wasted per passenger-hour.
Third, cars have highly flexible usage. Planes and trains are generally moving people or cargo to approximate destinations, and travel to places design to store them. Cars spend lots of time arriving in storage-free areas - what if your car dropped you off at work and drove to a garage outside the city? And they do lots of last-mile transit - what if your car could go pick up your groceries or dry-cleaning without you?
Broadly, I think there's way more safety and financial incentive for 100% driverless operation of cars than any other mode of travel.