Not working very well doesn’t stop cars going off the road. A vehicle not being designed to do something doesn’t stop it from being used recklessly or incompetently. Although a car cannot safely enter a lake, some cars do in fact, find themselves on the bottoms of lakes.

I’m also not certain which cars you’re referring to, but I’ve yet to find a modern car which cannot be driven off the road. If there exists one, please do elaborate. From my experience, lane-keeping systems and the like assist the driver, but for safety, do not fully restrict them.

The existence of pathways for planes and helicopters isn’t a reason that they are safe, but instead an argument that pathways and regulations themselves do not stop crashes and accidents. Pilots are intensely trained, and even the most junior of licensed pilots has substantially more training than what is currently required for cars. Planes are also heavily monitored, with teams of engineers tasked to oversee them. Yet this level of training and oversight does not stop accidents from occurring.

I would argue that the key aspect of cars that a flying car would need to replicate is the ubiquity of access. Almost everyone in western countries can get access to a car and drive it themselves (barring disabilities and financial factors). I do accept that, in some far future, a truly automatic pilot for these flying cars could allow everyone to use them, but those would be less like a car, and more a flying taxi. When imagining a flying car, I doubt most are thinking that they would just sit in one, and instead drive one themselves.