I know for some, it's the ridiculous cost of insurance. Especially if your kid is driving your nice, new more expensive car. As a teenager, I got placed on my parents insurance as the primary driver of their third car. Liability only since the car wasn't worth putting full coverage. Today, it's rare to see someone giving their teen an old beater car to drive. Cries about 'safety' and 'crash test rating'... my generation died like men in crumpled metal coffins (/snark). Either way, I know even that was quite high for my parents insurance.
I know my insurance is pretty cheap today due to a pretty good record. My wife's accident didn't even bump our rates up. But, I dread when my daughter hits 16 and gets her license how much it would cost to insure her in whatever vehicle she might drive.