Let me explain a bit about those raiders.
Raiders are a wandering group of Survivors that take from others to keep themselves safe. From what I hear they never tend to stay in one area for long. Whether it's because they made enemies or to avoid infected I have no clue
They tend to get their stuff from targeting and raiding people whether that be shelters, other groups, etc.
They have themselves equipped for combat and traveling, they're like those "post-apocalyptic gangs" but considering what this world is that's on par, They just don't have vehicles.
Most of the vehicles have been worn down by nature, and driving something like a car would be too risky anyways given they could catch the attention of Infected, the eyes or anything else outside. That risk is just my opinion though. There's probably still some functional vehicles inside buildings perfectly operational somewhere, not that you'll see me ever driving a car