The autopilot in modern planes does a lot more than just keeping the plane flying in one direction. You can get a plane land on autopilot (well up to like 1 or 2 minutes before touchdown). You can make it increase altitude automatically at a certain waypoint, decrease to a different flightlevel at a certain waypoint, and adjust speed. It monitors pilot input and intervenes if the pilot tries to do something "stupid." Basically, an autopilot in the planes is more than just a cruise control for headings. It can fly from point A to B with some intervention (like changing some data based on what ATC gave you).

The cruise control in Teslas cannot (yet?) drive you from point A to point B or even keep you driving in one direction forever without you holding the steering wheel occasionally or even hitting other objects. It can (just like cruise control in other cars) warn you if you are about to do something stupid (e.g. leaving the lane, about to hit an object etc.).

This is like people using foglights in cities at night to see better. You can use it for that, but it isn't made for that purpose. A light meant for FOG or other scenarios where the car itself is hard to see and not for you to light up everything in front of you (as it is quite bright if not used in fog).