> Cars are not planes.

Yeah, but planes aren't generally any better at this either. In almost everything except jet aircraft and especially jet airliners, autopilot can be as crude as "maintain this heading and altitude"; i.e. you still need to manage your speed (the engine throttle lever) yourself, including in response to changes of wind direction or intensity.

Then we get to airliners, which have flight management computers. You can give your flight plan to that, and they will even calculate optimal speeds along the way, which the autothrottle can maintain for you. This allows you to, for example, know when the optimum descent point from cruise is; too early and you need more fuel to maintain altitude when you get down to where you need to be, too late and you need to turn more to bleed off more altitude.

The more advanced flight management computers are also loaded with a complete navigation database that includes e.g. speed and altitude restrictions at each waypoint, so they know, for example, that you shouldn't descend below 12,000 feet or be doing more than 260 KIAS at <waypoint x>. This is basically what's on your charts, but it's still up to you to verify that you're complying with the chart, even if you let the flight computer handle it.

Then we get to the autopilot itself. The autopilot on a modern airliner doesn't mean the plane can "fly itself". They still need to be told what to do, and told correctly. If you take off from Paris and head south-east and tell it to maintain 8,000 feet, it /will/ fly you straight into the side of the Alps. The Ground Proximity Warning System will tell you that you're in imminent danger of smashing into a mountain, but the autopilot will not react to that. If you tell the autopilot to descend into the ground it will. If you tell the autothrottle to fly too fast you will damage your aircraft. If you're doing 280 KIAS in a 777 and you throw out flaps, the autothrottle will not slow the aircraft down, and you will damage them. If the autopilot is about to fly straight into another aircraft, the Traffic Collision Avoidance System will tell you that, but the autopilot will not react to that either.

Note that some of this doesn't apply to Airbus aircraft, as Airbus thinks they know better. Whether that's a good thing is up for debate; it did result in, for example, Sullenberger not being able to make an optimal ditching approach into the Hudson, because the Airbus thought his nose-up input would stall the aircraft (which it would have eventually, but he was only a few feet above the river anyway, so that's inconsequential). Then there was Air France 447, where the usually fastidious Airbus flight management computer would usually prevent the pilots from making input that would cause a stall, but because their speed sensors were not working, the computer didn't do that. The first officer held that plane in a constant stall for their entire 38,000 feet descent into the ocean, probably because they were relying on the system to not let them do that. The ground proximity stuff also doesn't apply to some fighter jets, which won't let a pilot aim the aircraft at the ground (because it could be because the pilot has passed out during a high-G manoeuver). Still, the vast majority of it applies.

The point is, these systems require the same kind of checks, instruction, and constant supervision that Level 2 autonomous driving systems do.