I'd say, it's probably easier to hack individual humans, but - like everything in computing - hacking algorithms scales well, while human factors generally don't.
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.
https://www.computing.co.uk/ctg/news/3020901/dhs-team-manage...
When you have 100,000 self-driving cars on the road, and someone figures out how to hack them, and drive them simultaneously into crowds, that will be a BIG BIG problem.
And this problem isn't just about self-driving cars. It is a fundamental issue with all algorithms. It's a lot harder to hack humans to do bad things (although it can be done with sustained messaging and propaganda). But algorithms can be compromised and exploited.