Survivor was a bout a really bad flu. So mostly everybody died in their beds/hospitals/homes, rather than being out and about. As a result, there's a lot of properly parked cars in traditional spots, rather than stuck in traffic and died situations.
As such, leaving your car door open makes it the most obvious one to take if somebody else comes up, and it signals to possibly unfriendlies that somebody alive is here now because that one car over there has an open door, and none of the others do. Which leads to somebody touching the hood and confirming that the car had been running.
