This. And though the book didn't get into it, the tank might be able to move but it could be completely surrounded. Zeke would swarm like slow ants on a carcass. What does the tank do when there are a hundred of them clinging and climbing on the tank? The Battle of Hope lasted almost a full day. That was a planned and coordinated fight that had tens of thousands of zombies. How long could a swarmed tank last with no visibility due to carrying a swarm? How close is support when that many zombies are around? You can't just go shooting them off or you might hurt the tank.