The lore of the game is that there was considerable instability before the zombiefication became known, with lots of riots and police brutality.
Anyone arrested/killed/injured on those would mean a car in decent condition on the streets somewhere.
The zombiefication wasnât that fast, but plenty of people still got caught before getting to safety (or to their vehicles). There were also many survivors who fled to the FEMA camp or similar, who might have piled up in buses to get there rather than go on their own.
In my experience, itâs easy-ish to get one or two cars that can run after checking, say, a parking lot or half a town if you have a screwdriver, but most of those cars wonât last you more than 2~3 trips before breaking down, less if you ever use it to dispose of zombie groups.
Usually, you canât get them running again without a few tools, some scavenging, some mechanics skill (2~3), some zombie-clearing and some surplus food to grind mechanics if necessary.