Because the driver is less likely to pay attention with a semi-autonomous system than driving manually.
In manual driving, you have every incentive to pay attention since the car cannot drive you when you are not paying attention. But in semi-autonomous mode, you have less incentive to pay attention because the car can still "drive" you when you are not paying attention. In fact, you can go miles with the car driving for you and you did not have to do anything. It can lull you into a false sense of security. So yes, it is potentially more dangerous, because you will pay less attention with semi-autonomous driving that lacks a reliable driver monitoring system than if you were driving manually.
makes logical sense, incentive is there. But lots of humans dont follow that when driving manual. A large amount of people, despite this incentive to stay alive, are chronically texting and fiddling their phones. If you go on AP and take quick glances (safely of course) into cars next to you a scary number of people are doing a wide variety of activities, manually driving. Sometimes I’m pretty impressed with their peripheral vision.