Ah, but fortunately we have anti-vandal-bots, too, automating much of that work. If necessary we could run a lot more of them, tuned to catch a lot more vandalism, but it's not a big enough deal (yet, anyway).
A half-competent vandalbot writer could write a bot that the anti-vandal bots wouldn't spot. Virus scanners need to be constantly updated to keep up with new viruses, our anti-vandal-bots would need to be constantly updated just the same, and the damage that could be done before the update is finished would be too great.