The issue is one of definition. I don't know why people associate autopilot with a definition that isn't autopilot.
An autopilot is a system used to control the trajectory of an aircraft, marine craft or spacecraft without requiring constant manual control by a human operator. Autopilot does not replace human operators. Instead, autopilot assists the operator's control of the vehicle, allowing the operator to focus on broader aspects of operations (for example, monitoring the trajectory, weather and on-board systems) [0]
The "why" is not really relevant here, the general perception is. The fact is, if you say "autopilot" and "self driving capabilities" to most people, they will infer that the car does not need a driver to be operated safely, which is definitely not the case.
Because it is being sold to customers in a different context. Autopilot for planes does not need to take into account other planes (flight paths are separated) to the same degree as cars do (where other "obstacles" constantly zip next to you).
An autopilot is a system used to control the trajectory of an aircraft, marine craft or spacecraft without requiring constant manual control by a human operator. Autopilot does not replace human operators. Instead, autopilot assists the operator's control of the vehicle, allowing the operator to focus on broader aspects of operations (for example, monitoring the trajectory, weather and on-board systems) [0]
[0] https://en.wikipedia.org/wiki/Autopilot