Most likely the combine didn't wan't them having cars, seeing as they would pose a security threat. Another thing is that most of the human population were in closely nit cities, they had no reason to go to other parts of the city on trips that would have required a car, the combine would have thought such trips to be for malicious intent. They also wouldn't be used to go between cities, as there are already trains laid out for that.
That's why the only working cars you get are managed by rebels.
Of course, there's also the factor of the all-too-scarce gasoline.126.96.36.199 17:32, May 1, 2010 (UTC)