The answer is enforcement: cars were much easier to steal in the past, and somehow we managed to deter theft anyway. But in many US cities if your car gets stolen the cops won't even pick up the phone. They openly admit there's next to no chance of recovery, and rarely help you even if you have some kind of tracking solution.

If you do manage to track down your car, you won't get a timely enough response from the police to hold folks who stole it accountable. Most of the time they'll simply claim that they procured the car from someone else, and the police don't push the issue. Same with bicycle theft.