> The thing is: Cruise control maintains the speed without human supervision.

At which point you would immediately veer off the highway and smash into an overpass, so no it doesn't.

It maintains speed without human involvement, which is the same thing lane keep assist does. It steers the car to keep it in the lane. Presumably even if the lane goes out to a pier and dumps the car in the sea.

Cruise control maintains the speed as long as it is physically possible. You don't need to supervise that it maintains the speed, it is able to do it on its own. It is (usually) even better than a human in maintaining the speed. Of course you still need to control the steering wheel or brake for obstacles -- but that's nothing cruise control promises to control.

On the other hand, lane assist is good, but it is not good enough to keep the lane without supervision. It can get confused by strange markings, obstacles, construction work, weather etc. It might fail to keep the car in its lane, although it is easily possible for a human.