Zombies are obviously just crisis actors.

50 ft wide tractor implements and tank treads are great in a blank, open environment, but your 50ft discer is not making it into the city, where the highest concentrations of people, and thus Zeke, will be. How do you clear a building with a vehicle? Ram it with the tank? Now you have maybe killed anyone inside who wasn't dead, and made a pile of difficult to traverse rubble out of a difficult to produce, possibly useful building, and no idea if there is anything inside waiting to infect anyone who starts trying to do the work of clearing it for human use, whether actively in the form of trapped zeds who can wait for ages or passively in the form of juices coating a sharp piece of rubble or biohazard remains getting into a cut. The danger with slow undead isn't in the open with the full light of the sun to reveal them. It's how they can surprise you, reanimating in a closet you have to clear with no space to backpedal, wandering off into the woods only to wander back once you think the area is clear, the scared people who are hoping it's just a regular infection and not wanting to be kicked out of camp so they turn in the night and suddenly they're inside the perimeter, next to where people are sleeping, the one that turned after crawling into a water tank to hide, and now that seemingly normal water people used to fill their canteens because they haven't seen clean water since the outbreak shut down the water company is turning a dozen people, seemingly at random. Small outbreaks can be handled. Once it reaches a certain size, though, things break in weird ways.