It's because morality (or the lack of) can have an impact on society as a whole, where having an auto accident only affects a very small amount of people involved.
I know I don't have to tell you this, but a fatal car accident does affect many people. From the families and communities that lost productive and appreciated members, to the emergency workers, auto manufacturers, and insurance people involved. A car crash can leave a bigger dent on society then you may think.
If politicians and the public at large were actually concerned about morality (or lack there of) then many things about our current day society would not be as they are. "Freedom" comes with certain concessions, namely that we don't impose our own beliefs and values on another member of a free society (to a point, of course). What we consider to be immoral may very well be an accepted and even revered tradition in someone else's culture, and of course the opposite may be true as well. Besides, I've never really thought of smoking a cigarette as "immoral." It may be unhealthy, but then if that's the logic behind it's age limit then there should also be an age limit on fast food. It's unhealthy and addictive, and eating to much of it will cause serious health problems, so it's hard to imagine why one is restricted.
Also, it is hard to imagine how a 16 year old teenage boy who likes to look at a playboy can have a larger impact on society than a three car pile up. And even conceding that, it's still ridiculous that it's arbitrarily ok to be as immoral as you want when you're 18 but not when you're 17, or even 16. Adults, the people who shape and determine the society we live in, can indulge in every legal immoral pleasure they so desire
without having a negative impact on society, but a 16 year old kid can't? Sounds like a bit of an illegitimate double standard to me.
I'm not in favor of abolishing age limits on things like cigarettes and pornography, but one must admit it is rather peculiar how, as Strubes pointed out, one is trusted with a 2000 pound vehicle or, as boomsickle pointed out, one is given a gun and trained to
kill another human being, before they're allowed to have a beer.