Well, that's why zombies AREN'T found that much in real-life ecosystems, mainly because they're too good at taking over everything and there's nothing left to take over. It has to find the right prey that can match reproduction rates with infection rates. In that ecosystem, there wasn't a bot that could do that. Well, except for the Caterpillar Chloroplastus, but it already had a little "colony" going on to defend against the zombies.