Doing a security analysis of a car is a complex task that most laymen cannot do, so the argument for laws and compliance among specialists is quite reasonable.

Put it another way: if it's all market based, your choices in buying just get 2x more complex. Is this car easily unlocked? Do the brakes often fail? Can it resist a collision?

Or even another thought: since most people don't think about security, companies will flood the market with insecure cars. Want a secure car? That would be niche and cost you double.