That's not entirely true, there are situations where they do disengage automatically (e.g. due to missing/conflicting sensor information). By the nature of the thing it is a) rare and b) the pilots have more time to react because it won't be a "you're going to hit a truck in 1 second" situation unless something went badly wrong before. But there is still some risk to the handover situation making it more likely pilots misinterpret the current state of the plane or do not realize which automatic support is still active (not just autopilot, also other avionics support (e.g. if I remember correctly one aspect around the Air France 447 crash is that the pilot likely did not realize that the plane had disengaged support systems that would keep him inside a safe flight envelope in normal circumstances).