Why does the first guy to see the invading monsters always get out of his car and run away? Unless he's is an Olympic sprinter or driving a Yugo, his car's top speed is at least double his top speed on foot.

Depends on the location. Since vehicles only have limited areas in which to travel, there's likely to be some quick bottlenecks that make it harder. Now, done well, you'll be away from those bottlenecks and have put more distance between you and the threat, but that's not always the case. On foot it's possible to get lost in the shuffle - kind of like schools, herds, and the like. Plus, a human is much easier to hide than a human in a car.