They're not really autonomous, they're mostly automated.
I'll lift this description from Wikipedia despite being written for cars (emphasis mine):
Autonomous means self-governance. Many historical projects related to vehicle autonomy have been automated (made to be automatic) due to a heavy reliance on artificial hints in their environment, such as magnetic strips. Autonomous control implies satisfactory performance under significant uncertainties in the environment and the ability to compensate for system failures without external intervention.
While most autopilots will fare fairly well under "significant uncertainties in the environment" (read e.g. "severe turbulence"), they are not equally capable "to compensate for system failures without external intervention". Even the Airbus' autopilots revert authority to the pilots if the system degrades.
Even knowing the destination airport, they need a pilot to insert the flight plan into the flight computer, they cannot compute it by themselves.
They cannot take evasive action in case of dangerous situations, but they can only warn the pilot (see EGPWS and TCAS RAs).
They cannot even properly recognize bad weather (say, a thunderstorm) and eventually plan diversions around it.
They could not land an aircraft by themselves, as they generally lack control over the aircraft configuration.
What they can do is blindly follow the flight plan given, so they are automatic, but not autonomous.