When shit goes wrong for a car (such as Waymo) you just stop. Now, that's not trivial, but it's also not very difficult, I expect most of it can happen even if the Waymo hardware itself were suddenly destroyed, rolling along is the hard bit so not doing that isn't too difficult. Everybody aboard can just leave when it stops moving.

In contrast when shit goes wrong for a plane we've got a big problem. Just stopping will definitely kill everybody, even from a modest altitude at a very low speed suddenly plummeting to the ground will straight up kill you. So, we want to land, albeit maybe we have to "crash land" destroying the vehicle to perhaps save its occupants.

You can buy (and indeed to some extent you can even retro-fit) emergency auto-land for small planes. Once engaged, or if set to do so automatically upon pilot failure the plane will figure out where it is (using GPS), pick the emergency radio frequency and announce the problem and its intended solution (I am a machine. My human pilot is incapacitated. I intend to fly to X location and land there. I am not listening to you and cannot understand you) and then it will fly to a chosen place and attempt to properly land the aeroplane, broadcast on radio that this airfield is now closed (this aeroplane is parked on the landing strip so you can't use it!) and then switch off.

Maybe the pilot is still alive and human medics can rush them to hospital. Otherwise maybe there are passengers who have been saved. In any case at least the aeroplane is now on the ground where humans can easily take over e.g. moving the plane so the airfield can re-open.