This is only because Tesla changed the meaning with its "fully self-driving but the government won't let us" marketing lies.

In the common parlance, saying "someone is running on autopilot" means they are going through the usual motions but not really paying attention to what happening or responsive to surprises. Pretty close to the aviation meaning and what Tesla car does.

I believe the point is nobody expects planes, where a layperson would know the term autopilot from, to be left unattended by the pilots. So why should this be assumed on the road?

Because human nature. No matter what they call it, the better it works the more the human driver will relax and let the machine take over. No matter how many times you warn them that that's inappropriate or unsafe.

Really isn't a hard concept to understand.

Autopilot means that the plane flies itself. Pilot is there in case it catastrophically fails.

You mean like how the driver is in the Tesla for if/when it catastrophically fails?