At cruise (really, above 10,000 feet or so, some airlines even allow it as low as 5,000) this is pretty much all they're doing anyway.
At SOME airports. They need a certain level of ILS in order to land autonomously, and this is not available at all facilities. There could also be a technical issue with the plane that would prevent this from happening (multiple ILS systems in the plane need to function for auto-land). Both Mentour Pilot and Captain Joe have done videos on this, I'm fairly certain.
As for the idea of a single-pilot cockpit - just think back to some crashes that turned out to be a result of a mental break by a pilot, one pilot leaves the cockpit for the restroom then the remaining pilot locked the door and wouldn't let them back in as they flew the plane into a mountain. Those resulted in regulations requiring at least two people in the cockpit at any time during flight, even if one was a cabin attendant. I guess those regulations don't mean anything?
IF this were implemented - to address the issue of the pilot becoming incapacitated they could implement something like the 'dead man's switch' in fishing ships or locomotives. They have to press the button periodically to confirm that the person at the controls is awake/alive, failure to press that switch would result in an indication to ATC that something is up.
BUT - what if the single pilot has an urgent need to use the restroom, a 'code brown' situation?
There are many arguments to NOT do single pilot operation, and I believe US FAA regulations will not allow it, as they are written right now.