According to the latest Federal Bureau of Investigation (FBI) report:
There were an estimated 699,594 thefts of motor vehicles nationwide in 2013
In the U.S. one vehicle is stolen every 45.1 seconds
More than $4.1 billion was lost nationwide to motor vehicle thefts in 2013
In fact, 73.9% of all motor vehicles reported stolen in 2013 were automobiles and the average dollar loss per stolen vehicle was $5,972.
Source: FBI Uniform Crime Report (2013)
Most car thieves are part of extremely organized crime rings and know how to steal any car they want. Locked doors won't stop them and security systems don't scare them off.