I had a car with level 2 self-driving, and it was amazing for the commute, going down from SF to the valley and back. It removed a lot of cognitive load from the boring highway driving, because that part is so predictive that the car could handle it. So that's a big partial win for systems like that.
But if you have to watch and be ready to take over at a moment's notice... it doesn't really remove that cognitive load? Unless you don't watch and aren't ready of course, but that's not really the point.
Except in highway driving, the amount of weird stuff that can happen is very, very limited, compared to city driving. It's predictable, and you learn what your car can handle, and what it can't handle.
I assume from your comment that you haven't actually spent a lot of time in a car with these features, but the best way to describe is that it's like driving in grooves. You're driving, but you don't have to actively work to keep your distance to the car in front of you, stay below the speed limit, or to keep your car centered in your lane. Just like with an automatic gearbox you don't have to actively work to keep your car in the right gear, the car does it for you. Same principle here.
