Experience. Indirectly linked to age. When you have been driving for a number of years and have experienced how quickly and unexpectedly things can happen while driving, it's hard to completely put your life in the hands of an automated system.
I think it's more fundamental. I trust a computer more to react quickly to something unexpected than I trust myself to. Maybe I'm biased because I spend all day programming and poking at computers and almost none of it driving.
I'm in my early 30s, and yes I also trust a computer to react better than me in the situations it has been trained for. The problem is for situations outside of the the normal, which based on my experience I'd say is what causes most fatalities in road traffic accidents. The fact that these are closed systems doesn't inspire confidence either - I wonder whether regulations will change this later, to require third parties to evaluate the system.
If I were to buy a new car today, I'd for sure look for one that has some of the modern safety features like lane keeping and automatic emergency braking. Dynamic cruise control would be great for long journeys. But I wouldn't feel safe riding in a car with what Tesla calls 'autopilot' capabilities, or more complete self-driving like Waymo - although to be fair in a city due to lower speeds the risk of injury (to myself) is going to less than on a highway, so I'd actually say I'd feel safer in Waymo than a Tesla.
Dynamic/adaptive cruise control is a really nice safety feature which I use whenever I can. I guess the 'autopilot' capabilities is just another form of that, but I'd still be hesitant to be hands off and read a book.
