Because it's happened before, has been mentioned, and we're really not sure if Rob has ever done anything about it. Whenever it happens it eventually dies back off again, and I don't recall there ever being confirmation as to whether that's been due to action from Rob, or just dumb luck of the bots getting "tired".
Many computer-savvy forumites have made suggestions of very quick fixes that have been summarily ignored so far as I can tell. And now with a new website in the works, I'm pretty sure Rob won't make any changes to this one now.
EDIT: Removed a portion bashing the webmaster because I realized that the webmaster Harknell isn't the guy I was thinking of who got fired for poor conduct.
"I'm afraid I don't understand. And also afraid that I do."
Two guys with basically the same name in a discussion about a character getting cloned.
There's gotta be a good joke in here somewhere.