Driving a car is different from 'taking over', if you drive a car you are alert because you are controlling the vehicle, being prepared to 'take over' is like having to wait for something out of the ordinary to happen and people are notoriously bad at such tasks. Their attention will be sharp for a little while and then drop off to levels where if something requiring intervention does happen they likely will have very little situational awareness, whereas if the same situation would happen when they were driving the car themselves they would likely be able to respond adequately because they were already processing all the relevant bits of information and would have accurate situational awareness.

If you're required to be ready to intervene that's about the worst possible way to introduce automated driving. And more to the point: the better the implementation the longer between 'interventions' the more likely that such an intervention will not be useful at all.