That's largely because of the need to salt roads in the canadian winters - yearly, excessive exposure to salt water drastically limits the possible lifetime of a car. The further away from cold weather requiring salted roadways you get, the longer cars last.
I'm really curious where the parent lives that people are investing so much in the longevity of a vehicle. By the time they get to 300,000 km even if they aren't rusty they are usually 20 years old and things like drivetrain start to go and the vehicle is not worth the cost to keep up the required maintenance.
Now to be fair, SoCal has a rather unique and essential car culture, so this is probably an outlier as far as "most" cities go.
Not all places use salt, and even then if you get your car washed and underbody cleaned it should help the longevity of the car.
Older cars, you keep them relatively clean and knock your boots before getting in so that the salt doesn't grind your carpets away, and replace things as they start to fail. A lot of older cars will just keep on chugging along.