Because it’s owned by one of the characters before things went bad. So they know it works, and most importantly they know it doesn’t have an alarm (loud sounds draw zombies). The problem with all those other vehicles sitting around is they know nothing about, they don’t know if they work, if the engine is load, if it has an alarm, how much gas is in it. Yeah a better vehicle would be nice, but you have to get it, and that entails a number of risks.
Zombies are popular because vampires got ruined. Vampires used to be a solid source of intractable evil for stories, vampires were bad, they were never not bad, and the hero character would always have to fight them. Now vampires sparkle. But zombies can’t sparkle, zombies will always be the enemy of the living. And they come with a great end of the world scenario that lots of people like to consider.
I agree with everything you said. I guess I just didn’t think that much about the choice of vehicle. And you’re right— Vampires are not supposed to be sparkly!