Probably because when you're off autopilot, Tesla drivers (including me) are much more alert and aware of what's going on compared to a normal car where you have to alert/aware the whole time and in so doing, tend to "zone out" when doing monotonous driving. I tend to use Autopilot for the monotonous driving parts .. and then take over myself if there is something that needs more attention (construction, weather conditions, crazy drivers, etc.)