There are multiple safeguards for planes that don't work for self-driving cars.

First, they're not physically inspectable by every Tom, Dick, and Harry. This means it's easier to obscure interfaces and other potential areas of attack. Not state-actor proof, but probably ISIS-proof. Second, they only get software updates while parked at an airport, so it makes MITM attacks harder (though not impossible). Third, they're in the air where death isn't a sudden movement away. Pilots can override the system and fly it manually. Fourth, they're heavily monitored with errant flightpaths reported to militaries around the world (to stop another 9/11).

We have none of these safeguards for self-driving cars and there are going to be hundreds of millions of them.

Pilots have the ability to manually disengage airplane autopilots, and more importantly the time to do so (on the order of minutes). It's very different in a car, where the difference between life and death can be a split-second reaction.

It has already been demonstrated non-self-driving cars.

https://www.bbc.com/news/technology-33650491

Not in-flight as far as we know, but a Boeing 757 was hacked in a proof of concept attack on the tarmac quite recently

https://www.computing.co.uk/ctg/news/3020901/dhs-team-manage...

The difference is the massive amounts of regulations concerning flight systems.