The problem is what to do if things go wrong...
"Autoland" has been available since the late 60's and was certified with a failure rate of 1 in every 10^9 hours of operation. Possible with hard wired systems and triple redundancy and necessary because the number of "tests" mounts up fast with the number of commercial aircraft (of a certain type) flying and taking off day in, day out.
But what happens if the sensors the aircraft is using to sense its surroundings fail? What happens if one of the engines fails? What happens if the controls fail but you can still control the aircraft through engine throttling (BTW following NASA tests that is now a flight computer mode on some A/C)? What happens if you can't get the undercarriage down?
Most of these are survivable throughout most of the flight (even an engine fail on takeoff within limits) but the system has to diagnose the fault and decide on a course of action..
Safe solution. Code detection tests and actions for all known (or at least probable) failures and trigger a re-evaluation if sensor data changes (but remember the sensor data may be the fault and the system may be operating normally).
Riskier solution. System that applies logic to the problem IOW a form of AI.
BTW NASA does this already for some of its probes and scientific satellites The trouble is can such a system a)Diagnose what's happened b)Develop a plan to mitigate it c)Implement it fast enough to save lives?
Fixed altitude/heading/speed cruise you could do with an 8 bit micro controller.
Or you just do what they do with microlights and fit a whole vehicle parachute and just fire that.