It's not autonomous because it is rote automation.

It does not have logic to deal with unforeseen situations (with some exceptions of handling collision avoidance advisories). Automating ATC, clearance, etc, is also not currently realistic (let alone "the easiest part") because ATC doesn't know what an airliner's constraints may be in terms of fuel capacity, company procedures for the aircraft, etc, so it can't just remotely instruct it to say "fly this route / hold for this long / etc".

Heck, even the current autolands need the pilot to control the aircraft when the speed drops low enough that the rudder is no longer effective because the nose gear is usually not autopilot-controllable (which is a TIL for me). So that means the aircraft can't vacate the runway, let alone taxi to the gate.

I think airliners and modern autopilot and flight computers are amazing systems but they are just not "autonomous" by any stretch.

Edit: oh, sorry, maybe you were only asking about the Garmin Autoland not being autonomous, not airliner autoland. Most of this still applies, though.